After a behind-the-scenes political battle, it took over the lead role from the police last year in fighting terrorism, particularly the IRA. Senior police officers are concerned that further MI5 expansion will lead to a second, secret police force operating without real public accountability.
With the Cold War's end, the 2,000-strong agency, which is not directly accountable to Parliament, has been seeking new roles. It has pushed into the fight against drugs traffickers, and there are claims that it is involved in industrial espionage.
Sources at the National Criminal Intelligence Service - a new FBI-style criminal intelligence gathering body - say the next item on MI5's shopping list is fraud. In a recent Radio 4 interview, Kenneth Clarke, the Home Secretary, referred to MI5's role in fighting 'organised crime'.
Home Office sources say Mr Clarke wants to diminish local control of the police and drastically reduce the England and Wales forces to a handful, moving towards a national force controlled by the Home Secretary and funded from central government. Derbyshire's Chief Constable, John Newing, is one of several senior police officers concerned at these developments.
He said: 'Putting MI5 into policing is a retrograde step. MI5 have been set up for a specific purpose, dealing with terrorism and espionage. They are not police officers.
'This is centralisation by stealth. There are a number of areas like organised crime and international crime which need to be dealt with at a national level. But there also needs to be accountability. The Home Secretary should debate exactly what's intended by these moves. None of these things are being laid on the table and discussed.'
Eighteen months ago the Cabinet Office reviewed intelligence gathering operations against the IRA and recommended that the existing arrangements continue with the police in the leading role. MI5 was not satisfied, and successfully argued for another review, set up weeks later.
While that was being conducted last April, an internal Scotland Yard document admitting ineffectiveness against the IRA was mysteriously leaked to the Irish Times. Days later MI5, which is known as 'Box 500', got the decision it wanted.
A senior Scotland Yard officer said: 'The product of terrorism is a crime, and we arrest people and take them to court, where the courts examine the surveillance and the evidence. MI5 officers have traditionally been faceless. You don't know how good they are at giving evidence, or in conducting investigations in ways which will impress a court.
'If they had to change to operate in that way, like police officers, then you'd be as well using police officers in the first place.'
In private discussions involving police, MI5 and the Home Office it has been conceded that the distinction between intelligence gathering and police operations is blurred. One very senior police officer described it as 'artificial, and meaningless'.
The Metropolitan Police has been giving courses to MI5 on how to present evidence in court in anticipation that it will become more involved in court cases.
MI5 is also believed to have stepped up bugging and tapping work in the commercial sector. One private security firm director who sweeps company offices for rivals' bugs said: 'Increasingly we are finding equipment with MI5's trade marks all over it. MI5 is extending its monitoring of firms which do business abroad.'
The Government disguises the number of official taps. Warrants may be as low as 500 a year - but each may cover a whole organisation, or all telephones to which an individual has access.
Insiders estimate that as many as 35,000 lines are tapped annually, the bulk for the Security Services and Special Branch. Since 1980 the number of specialised engineers employed by British Telecom to mount the taps has increased by 75 per cent.
A new high-security installation at Oswestry, in Shropshire, will make it possible for even more taps by computer, without engineers having to leave the building. Individual lines can be intercepted instantly and relayed to a listening centre anywhere in Britain.
It will also allow 'treeing,' making tapping much more effective. If target A makes a call to B, and B then calls C, each will be tapped instantly from Oswestry.
MI5, MI6 and Special Branch officers also have regular briefings at Martlesham, British Telecom's research establishment in Suffolk.
Meanwhile, dozens of the 530 Special Branch officers based at Scotland Yard face a move back to ordinary duties after a secret internal review. Although the decision will be taken by Paul Condon when he takes over as Metropolitan Police Commissioner next week, SB officers are angry that another defeat in their battle with MI5 will cost them jobs.
Barrie Irving, director of the independent Police Foundation, said: 'To allow MI5 into organised crime, is allowing them into the policing arena. Police performance is being evaluated more than ever before.
'Is MI5 being brought in as a secret force to do the sort of things that the police used to get away with?'
Such an enlarged role for MI5 will rekindle the row over its scrutiny by Parliament. In spite of demands for a select committee to carry out that scrutiny, John Major will give the task to a group of privy councillors, covered by secrecy rules.Reuse content