Time to protest is when you're in office

GCHQ is an an expensive toy for ex-ministers to play with, and compromise at their whim. GCHQ certainly costs a lot of our taxpayers' money, the lion's share of the declared approximately £1billion spent on the three main intelligence agencies which receive their funds through the Single Intelligence Vote.

We also know from evidence given during the Matrix Churchill inquiry that the greatest proportion of the 40,000 separate items of intelligence circulated in Whitehall every year comes from the codebreakers at Cheltenham.

We know, too, from the Falklands conflict that GCHQ has saved many lives. The best information about the precise size of the Argentine occupation forces, and their deployment, came in the form of a decrypted rations requisition which identified the exact numbers the Task Force would be challenging.

Yet such sources are easily terminated. In 1927 the British government unwisely disclosed that it had been intercepting and reading the Kremlin's traffic. The source dried up immediately. In 1948 Kim Philby betrayed the VENONA programme which had allowed GCHQ and its American partner to glimpse the NKVD's plots.

While Philby's purpose was deliberate, Ted Rowlands in April 1982 seems only to have wanted to shock when the ex-Foreign Office minister of state indiscreetly revealed in a speech in that "we have been reading the Argentine diplomatic telegrams for years".

Such disclosures, planned or spontaneous, can have devastating consequences.

More recently, a single remark from a US senator, commenting on the NSA's ability to monitor Osama bin Laden's satellite telephone conversations, prompted the terrorist leader to switch to another method of communicating with the al Qaeda leadership. The time for ministers to protest about dirty tricks is when they are in office and are in a position to influence government policy and change the targets for interception selected by the Joint Intelligence Committee. GCHQ is an instrument of political power and is directed by the Cabinet through the JIC.

Even resignation from office and a public statement is a more honourable strategy than post facto disclosures from the backbenches. Although far from ideal, this tactic at least gives the agencies the opportunity to undertake some damage limitation before the targets become aware of the weaknesses in their systems that have been exploited so successfully, and at such massive cost.

Of course, interception of such diplomatic traffic does not necessarily signify hostile intent. GCHQ has a major role in testing Britain's communications channels and ensuring their integrity. Such security exercises are also routinely conducted on the systems employed by those with whom we are in confidential contact, to prevent leakage through a third party. Who is to say that this was not the motive on the occasion about which Clare Short complains?

Sigint (signals intelligence) is the largest investment Britain makes in the arcane world of espionage, and it is one that is easily compromised and wasted.

The sensitivity surrounding these sources is obvious, and it has taken two years of hard bargaining within the intelligence community for the official historian of the Falklands conflict to be allowed to publish the crucial intercept that proved the Argentine cruiser General Belgrano had received orders to execute a pincer attack on the British Task Force.

When this is released next year, it will be the first officially sanctioned publication of an intercept since 1927.

Characteristically, Clare Short has managed to short-circuit the well-planned strategy, with as yet unknown consequences for the professionals to live with, perhaps for many years to come.