Seven out of eight people arrested under Britain's terror laws since the al-Qa'ida attacks on America in 2001 were not convicted of a terrorism offence, figures released yesterday show.
More than three-quarters of those imprisoned were given sentences of less than 10 years and a half will be released in less than five years.
Between 11 September 2001 and 31 March 2008, there were 1,471 arrests under terrorism offences in Britain. Of these, 521 resulted in a charge of some form, with 222 people charged with terror offences, and 118 people charged with terror-related offences, such as conspiracy to murder.
Civil rights campaigners last night seized on the figures as more evidence to support concerns that police were using tough terror powers indiscriminately against mostly innocent people.
Shami Chakrabarti, director of the human rights group Liberty, said: "In free societies we arrest on suspicion, charge with evidence and convict when there is proof. These figures remind us that the overwhelming majority of those arrested for terrorism were not guilty of any charge and half weren't charged at all. All the more worrying that wholly innocent people may be held for a month without charge or indefinitely without charge under control orders – based on secret suspicions and intelligence alone."
The statistical report, the first of its kind, follows a series of high-profile cases in which suspects arrested and detained under the Terrorism Act have been released without charge. Last month it emerged that the case against 12 Muslim men involved in what Gordon Brown had described as a "major terrorist plot" amounted to one email and a handful of ambiguous telephone conversations. Eleven Pakistani students and one British man were freed after extensive searches of 14 addresses in north-west England failed to locate evidence of terrorist activity, according to security sources.
The new figures will also support concern that police use of anti-terror stop-and-search powers is alienating Muslim communities. Last year searches under the terror laws trebled.
Officers in England and Wales used Terrorism Act powers to search 124,687 people in 2007-08, up from 41,924 in 2006-07, separate figures released this month revealed. Of the subsequent 1,271 arrests, only 73 of those were for terror offences.
But the Home Office said the number of those arrested who were charged is similar to that for other criminal offences.
The Policing and Security minister Vernon Coaker said: "We know we face a real and serious threat from terrorism and the figures outlined today show 196 terrorist-related convictions between September 2001 and March 2008. This underlines the considerable success that the police, security service and intelligence agencies have had in disrupting terrorists and that the CPS has had in prosecuting these individuals. I am very grateful to them for their hard work in protecting the public and keeping us safe.
"Considering the evidence presented in these trials, there can be no doubt about the nature and complexity of the threat: aspirations to use a dirty bomb; the targeting of shopping centres, nightclubs and our transport infrastructure; the desire to inflict mass casualties on the public without regard to race, creed or colour; the aspiration to commit terrorist acts abroad; and the encouragement and support of terrorism both in the UK and overseas.
"That is why the Government is committed to investing in our counter-terrorist threat and wherever possible seeks to prosecute those involved with terrorism. Where we can't prosecute, we seek to deport, and where we can't deport, we seek to disrupt."
The figures show that almost 230 people are arrested for terrorism offences every year. But the number eventually convicted of a terrorism or terrorism-related offence is 13 per cent. The Home Office chief statistician Professor Paul Wiles said the conviction rate in relation to arrests was likely to rise as some of those charged were still awaiting trial.
The figures show that 46 per cent of those arrested under Section 41 of the Terrorism Act 2000 were held in pre-charge detention for less than one day and 66 per cent for less than two days.
After this period they were either charged, released or subject to alternative action, such as transference to the immigration authorities.
The maximum number of days police are allowed to hold someone under the Terrorism Act increased to 28 days in July 2006.
The Tory security spokeswoman Baroness Neville-Jones said: "One of the most powerful deterrents to terrorism is successful prosecution and imprisonment. The Government needs to allow intercept evidence in court so that real terrorists don't get let off for lack of admissible evidence."
The record so far
* Between 2001 and 2008 the conviction rate for terrorism or terrorism related offences was 13 per cent.
* In 2006/07 there were 488,100 arrests for 'violence against the person', of which 9 per cent, or just over 42,000, resulted in sentencing.
* Of the 32,100 arrests for sexual offences, just 6 per cent (5,054) of offenders were sentenced.
* There were 40,800 arrests for robbery during the period, of which 12%, or 8,862, of cases ended in a sentence.
* Nearly 50 per cent, or 44,500, of the 89,200 arrests for drug offences made in 2006/07 resulted in sentencing.
The Government does not keep comparable figures for non-terror offences where it is possible to trace an arrest to the final outcome. The figures are the result of The Independent's own research using statistics published by the Home Office and the Ministry of Justice.