Britain is almost alone in banning the use of intercept evidence in court - a prohibition that represents a victory for the intelligence services who have consistently argued that any such move would put their operations at risk.

But there is growing support across both the legal profession, civil libertarians and the police for the law to be relaxed on the ground that it would provide what Lord Goldsmith, the Attorney-General, today called a "key tool" in bringing major-league fraudsters and terrorists to book.

Civil libertarians also argue that the use of such evidence would mean that suspects who are now held indefinitely without trial under effective house arrest subject to control orders could instead be brought to court; and acquitted or convicted accordingly.

Anthony Arlidge, QC, a leading criminal barrister, said at a conference organised last year by Justice, the human rights group and Sweet & Maxwell: "Far better that intercept evidence is used in court than the autocratic procedures that are currently in place to deal with such cases.

"More genuine terrorists would be properly convicted and fewer innocent people would have their liberty unnecessarily curtailed."

Until 1985, the interception of communications by the state was outside the law altogether. But a European Court of Human Rights case called Malone forced a change in the law and put telephone tapping for the first time on a statutory legal footing.

However the courts were still excluded from the process: the Interception of Communcation Act 1985 which followed stated that no evidence could be adduced in court suggesting calls had been intercepted.

But as far back as 1996 there were calls for reform. Lord Lloyd of Berwick urged such evidence be used in national security cases but his proposal was rejected by the Government and instead it went on to to re-enact the prohibition in the Regulation of Investigatory Powers Act 2000.

Since, though, there have been mounting calls for lifting of the ban and even Stella Rimington, former had of M15, condemned the restriction as "ridiculous".

David Blunkett, at the time Home Secretary, also floated the idea of possible ways of using "intercept evidence" to bring terrorist suspects then held in Belmarsh prison under anti-terrorism laws to trial.

But intelligence authorities have argued that their methods and sources would be at risk of exposure. They also say that defence lawyers would mount "fishing expeditions" and prosecuting authorities put to huge time and expense going through hundreds of hours of irrelevant conversation.

A third argument, according to Pofessor Conor Gearty, in an article in the London Review of Books last year, is that much of the material would be likely to be inadmissible.

He says: "If this is true, it is important to know why. If the intercepts are too unreliable, too easily tampered with, to be usable in court .....how can they be deployed in any kind of intelligence assessment?"

At present, not only is the UK almost alone in refusing to allow intercept evidence in trials; it maintains the anomalous position in which legally-obtained foreign intercept intelligence is allowed as evidence, but UK intercept evidence is not.

Mr Arlidge said: "It makes the UK investigatory agencies appear excessively secretive and paranoid. Why is it that the UK agencies require the protection of anonymity when others, such as in the United States, do not?"

He points out that all defendants in serious cases know that telephone communications can be intercepted and traced. There is no reason to think that they will use mobile telephones any less if the evidence is admissible in court.

Safeguards, as Lord Goldsmith acknowledges, would be needed so that prosecutors were allowed to call such evidence but not compelled to do so.

But none of the voiced concerns are unsuperable. As Guy Mansfield, QC, former Bar chairman, said last year in backing a change in the law: "it is incorrect to say this evidence cannot be used.

"Material gathered by police surveillance is already used, with safeguards, and the same should be possible with security evidence."

The identify of parties can be proved by acoustic analysis and voice prints, without informant evidence, he says; nor need the means of obtaining the evidence be disclosed. Finally, if too sensitive, the evidence does not need disclosing at all."

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