Should we be as afraid as Mr Blair would like us to be?

The Prime Minister warned Britain last week that there are 'several hundred people in this country who are engaged in plotting or trying to commit terrorist acts'. His security services disagree with him, saying there are no more than a couple of dozen. So why the discrepancy? And how should we assess the threat? Raymond Whitaker and Paul Lashmar investigate

Few expected any surprises from Tony Blair's appearance on BBC Radio 4's Women's Hour on Monday. It was the third of three set-piece interviews with party leaders on issues such as child care, working mothers and the health service, and for nearly half an hour the usual pieties were uttered. Then the Prime Minister dropped an apparent bombshell.

Few expected any surprises from Tony Blair's appearance on BBC Radio 4's Women's Hour on Monday. It was the third of three set-piece interviews with party leaders on issues such as child care, working mothers and the health service, and for nearly half an hour the usual pieties were uttered. Then the Prime Minister dropped an apparent bombshell.

Speaking of the need for new powers to control terrorism, Mr Blair said: "There are several hundred in this country who we believe are engaged in plotting or trying to commit terrorist acts." He seemed to be implying that once the Government succeeded in getting its plans for control orders through Parliament, hundreds of British people could be subjected to such orders, preventing them using mobile phones, meeting certain people, or surfing the internet.

Yet this claim, far from leading news bulletins, caused little reaction. Most of the reporting devoted to his words emphasised that security officials considered them "sloppy". In their view, the number of people who could be considered a "serious" or "moderate" threat was no more than about two or three dozen.

What was behind this remarkable discrepancy? Downing Street later said the Prime Minister was talking about "people who there are concerns about ... Potentially they are planning and thinking about terrorist action".

The Prime Minister's namesake, Metropolitan Police Commissioner Sir Ian Blair, also laid the emphasis on "potential", saying: "I am aware of the fact that there are very many people who came back from the camps in Afghanistan and who are therefore potentially a threat to the United Kingdom ... I agree with the Prime Minister's assessment, on that basis."

But Mr Blair himself said a few weeks ago that only "a handful" would be affected by the control orders being proposed. The Home Secretary, Charles Clarke, says there is no need at the moment to put anyone under house arrest, not even those foreign citizens being held in prison without trial under the current anti-terror law, which is about to expire. In little-noticed evidence to a Commons committee last month, Mr Clarke said the number facing restrictions close to house arrest might not be "significantly larger" than the 17 people who have been detained without trial at various times.

Mr Clarke said: "It is quite possible by using a range of control orders to restrain people ... from being able to plot and plan terrorist activities." The number that might need close control, he implied, was highly unlikely to be anywhere near three figures.

So where does Mr Blair's "several hundred" come from? Some have suggested that it refers to the 701 people arrested between the 9/11 attacks and the end of last year under the Terrorism Act 2000. But half - 351, to be exact - were released without charge, some after dramatic swoops which triggered lurid reports of plots to bomb Old Trafford or release poison gas in the London Underground.

So far, 17 of the 701 have been convicted of terrorism. Another 119 have been charged, while 135 are accused of terrorist offences under other legislation, including murder and the use of firearms or explosives. Three people have been jailed for 20 years in two separate cases for planning bombings, and last week there was the chilling case of Saajit Badat, who pleaded guilty to a plot to blow up an airliner with a shoe bomb. Unlike his friend Richard Reid, serving life in the US for a failed bombing over the Atlantic, Badat never went ahead with his mission, but kept the explosive at his Gloucester home.

If the Prime Minister, like Sir Ian, was thinking of the 300 to 600 Britons thought to bear watching among the thousands who trained in Afghan camps run by al-Qa'ida and the Taliban regime, Badat fits the description. He spent two years in the camps, an unusually long time.

According to security sources, people like him are now the main area of concern: British-born Muslims who underwent military training and in some cases fought in Afghanistan, Bosnia, Chechnya or Kashmir, before returning to "radicalise" their younger, disaffected counterparts.

MI5, which leads the anti-terrorist effort, believes it identified and neutralised all al-Qa'ida terrorists operating in Britain even before 9/11. Immediately afterwards it saw the main danger as coming from North African exiles, most of whom were suspected of raising funds through crime for terrorist activities abroad. Having largely suppressed that threat, often through securing convictions for credit-card fraud, and having done its best to prevent al-Qa'ida operatives from entering Britain, the agency's focus has moved on to the risk of home-grown terrorism.

Yet the fact that Badat felt unable to go through with a suicide attack shows that the Britons who went through the hands of al-Qa'ida and the Taliban cannot all be seen as an undifferentiated threat. Security sources point out that many returned from the camps with their dreams of jihad shattered; others, like Badat, lost their resolve when they were absorbed back into British society.

Far from "several hundred" who are "plotting or trying to commit terrorist acts", the "hard core" is estimated to be far smaller, possibly 20 or 30 people. One source privy to a recent Security Service briefing said some 90 people in Britain were loosely organised into cells sympathetic to al-Qa'ida, adding: "They have had a lot of intelligence resources put into them."

While no one denies there is a serious threat, many question the tactics and timing of the Government's words and actions on the issue, such as the sudden, and still unexplained, appearance of armoured vehicles at Heathrow just before the Iraq war began. Last November a scare story about an alleged al-Qa'ida threat to Canary Wharf in London was fed to two Whitehall correspondents just before the Queen's Speech announced tough new anti-terror measures. This caused fury among security officials who said they had never seen a specific threat to Canary Wharf. "The security forces have been at great pains to avoid scaremongering, and then it seems someone in government circles has done exactly that, purely for short-term political gain," complained one.

"I have no idea what the threat is - that is the Government's prerogative," said Shami Chakrabarti, director of Liberty, "but they are not acting responsibly in their announcements on the level of the threat. It is not enough to say, 'I know something you don't know'."

So successfully has the threat been contained, in the view of some, that it should be possible to handle it within the existing criminal law. Conor Gearty, head of the Centre for the Study of Human Rights at the London School of Economics, is among those who believe that if evidence from telephone intercepts was admissible, terrorists could be dealt with in the courts. "Every other large country uses intercept evidence in this way," he said. "Britain is rather unusual in seeking to protect sources rather than using the evidence to convict people."

Support for Professor Gearty comes from the US, where some FBI counter-terrorist officials argue that terrorists should be treated more like criminal gangs and less like spies. If phone-tapping and electronic surveillance were organised to secure criminal convictions, they argue, it would be possible to use the evidence in court without divulging sources and methods.

Nicholas Baker, formerly the head of surveillance at HM Customs and an adviser to the Government, disagrees. "Very extensive Home Office trials, looking at how such evidence would have affected past trials, showed it would bog down the criminal justice system and make trials much longer," he said. "It would also jeopardise the close relationship between the security authorities and law enforcement agencies, which is the envy of many in the US."

But there was a more fundamental problem. "What terrorists might do is so terrible that you have to stop them before they do it, rather than focusing on gathering evidence after an offence has been committed," said Mr Baker. "The opinion polls show the public just doesn't want these people to operate."

A government paper sent to MPs last week said al-Qa'ida and its offshoots differed from previous threats, such as the IRA, because of their global reach, rejection of negotiation, desire to cause mass casualties and willingness to sacrifice their own lives. Questioned over the balance of defeating terrorists and maintaining human rights, Mr Blair said: "If there were to be a terrorist act, people would be asking me why I hadn't protected the civil liberties of our citizens, because they have a right to life, it is also a human right."

Lord Harris, a Labour peer who chaired the Metropolitan Police Authority for three years, insisted last week that the new control orders would be subject to "all sorts of checks and balances ... the Home Secretary would be required to follow a very precise legal process".

Government sources are indicating that it would take just one terrorist incident for the present uproar about house arrest, control orders and civil liberties to evaporate. Instead, as the Prime Minister hinted, the authorities would face furious criticism about its failure to prevent the attack. And with a nearby general election, Labour is bound to take that threat seriously.

A threat to the nation or the politics of fear?

Conor Gearty, head of the Human Rights Centre, LSE

"It brings politics to an end if we are going to be governed by the politics of atrocities that haven't yet happened: we might as well give the Government and the security services a blank cheque. The Government has in the past used nightmare scenarios to say it needs emergency legislation without which the country will be doomed. It elides with weapons of mass destruction. There is now an absence of trust about these assertions: 'I know what's going on, but I can't tell you.'"

Lord Harris, ex-chairman, Metropolitan Police Authority

"The reality is that there is a very substantial threat out there, one which is of a different order to any we have faced before ... We are dealing with people who are prepared to die and quite willing to seek enormous casualties. If there is evidence that is not admissible in court, but which suggests very strongly that some individuals are preparing to carry out some appalling atrocity, we must be able to intervene."

Nicholas Baker, ex-head of surveillance, HM Customs

"Some argue that if there are only 20 or 30 'hard-core' terror suspects in Britain, it should be possible to keep them under 24-hour surveillance. But that would probably take more people trained in covert surveillance than there are in all Britain's security and law enforcement agencies. It is not a practical alternative. The fact is that house arrest and control orders would be far cheaper. Surveillance and telephone intercepts are hugely expensive. They are almost a last resort."

Shami Chakrabarti, director of Liberty

"The Government doesn't want an informed debate about control orders. We have seen the opposition to the Government's proposals, even at a time of pre-election frenzy and fear about terrorism. In their view, proper debate and adequate scrutiny doesn't help. They have secret intelligence about terrorism and I don't: that is always the trump card of the government of the day. But it is a prerogative that has to be used wisely, not least in when they choose to reveal things."

FALSE ALARMS AND REAL-LIFE THREATS

The bomb plotter, 28 February 2005

Saajit Badat, 25, pleaded guilty at the Old Bailey last week to a plot to blow up an airliner with a shoe bomb. Forensic evidence showed the fuse was cut from the same length used by another Briton, Richard Reid, jailed in the US for his attempt to bomb an airliner. Badat will be sentenced next week. After returning to the UK from training camps in Afghanistan, he emailed his handler to say he had changed his mind, but kept the explosive at his Gloucester home.

The ricin panic, 7 January 2003

Police enter a flat above a chemist's shop in Wood Green, north London, in January 2003 amid reports it contained the deadly poison ricin. There were claims that a chemical weapons attack had been foiled. But the occupants of the flat, Samir and Mouloud Feddag, 26 and 18 respectively, were never charged with terrorist offences. When they appeared in court, the brothers were jailed for possessing false passports.

The fertiliser find, 30 March 2004

An officer in protective gear enters a house in Crawley, Sussex, in March last year, during a series of raids across the South-east. The operation discovered 600kg of ammonium nitrate fertiliser in a storage facility in Hanwell, west London. Later five men and a teenager, all of Pakistani origin and aged between 17 and 32, appeared on terrorism charges, including conspiracy to cause an explosion likely to endanger life or cause serious injury.

The soccer scare, 19 April 2004

Manchester United fans were searched in March 2004 after 10 Muslim men were held amid claims of a plot to bomb Old Trafford. But all were freed without charge - eight almost immediately, Rebaz Ali and Shadman Sophi after being held for eight days. Their only offence was loving Man U; they went to the next match as guests of the supporters' club.