Leading Article: Tinker, tailor, soldier ... police?

What is MI5 for? In the Eighties, most people thought it was there to monitor spies working in Britain for Communist and other unfriendly governments, with a sideline in spying on trade unions and disarmament groups. When the Cold War ended, MI5 had to reinvent itself. In her televised Dimbleby lecture just over a year ago, Mrs Stella Rimington, head of MI5, acknowledged that almost half her budget was going on counter-terrorism in Northern Ireland, and that significant resources were spent in infiltrating other terror groups working in Britain. Then came the Northern Ireland ceasefires.

Since that time it seems that MI5 has been trying to consolidate a role in tackling organised crime and in particular the drugs trade. This is the natural terrain of the Special Branch and Customs and Excise. Mrs Rimington insists that MI5 has skills that the other services need - infiltrating secret groups, liaison with secret services overseas and so on. Maybe so, but it is hard to see why the Special Branch couldn't learn to do these things just as well.

This is not simply a case of pointless duplication, brought about because Britain can't manage to close down government agencies that have outlived their role. There are deeper reasons why we should be worried by MI5's ambitions.

Spies don't necessarily make good police officers. They don't like appearing in court or being cross-examined on their evidence (unless behind screens with electronically distorted voices), because it blows their cover. This gives rise to fears about the reliability of their evidence. The British criminal courts can't afford any more miscarriages of justice.

The culture of MI5 is, understandably, that of a clandestine body: it fights dirty. Mrs Rimington has recently said that civil liberties are "almost inevitably" sacrificed in MI5's line of work. Intelligence organisations often set people up, and even involve their agents in crime, in order to secure their positions in the groups they penetrate. The public will tolerate these practices against foreign spies, but may be uneasy about their wider use. The courts have recently been critical of the police for using such tricks to obtain evidence. We cannot allow MI5 to do things we would not permit for the Special Branch.

Finally, MI5 is subject to much less public accountability than the police forces are. If spies are to become crime-busters, they should be subject to the same sort of scrutiny and openness, perhaps through a specific Security Policy Agency with powers similar to those of the police authorities.

The Security Services Act of 1989 does not define sufficiently clearly just what M15 can work on. It only requires that Mrs Rimington show a threat to national security. It is high time for more precision. We should not unloose MI5 on international organised crime until until new rules of engagement have been openly debated and agreed. Government has yet to satisfy the public about MI5's ethics, its role in the courtroom, the scope of its mandate or its accountability.