Dr Julian Lewis: The limited amount of time and the number of hon. Members who still wish to speak means that I shall endeavour to be as concise as possible.

Most of the debate so far, including the erudite contribution of the hon. and learned Member for Redcar (Vera Baird) has rightly focused on the legal issues which confront the Home Secretary and the Minister. I do not envy either of them their dilemmas. They must consider, in the domestic context, what is known in the international context as the problem of pre-emption. The Minister summed it up succinctly in her intervention on my right hon. and learned Friend the Member for Sleaford and North Hykeham (Douglas Hogg).

However, my aim is to consider the security issues that have emerged concerning the disclosure of intercept data in criminal trials and the recruitment of extra staff by MI5. Most of us would accept that it is fundamental to counter-intelligence work to maintain a strong element of surprise and to keep one's enemy guessing. It was therefore a little unexpected to read in the Guardian on Monday 23 February:

"MI5 has won the backing of Tony Blair to allow the product of telephone taps to be used in court cases".

Historically, it has usually been the other way around: politicians usually agitate for the disclosure of such material; security professionals tend, for very good reasons, to resist them.

That point has been illustrated repeatedly over the past 80 years. One can go back to 1927 and the famous raid on the Soviet trade delegation and the ARCOS Company. The Home Secretary disclosed the important techniques used by the security services, and we were unable to take similar action against Soviet dangers for many years after that.

As recently as the first attack on the twin towers in the early 1990s, the Americans revealed information about their ability to listen to mobile telephone conversations, which ensured that they did not get that sort of warning when al-Qaeda tried again, successfully, in 2001. The danger of such disclosures is not only that terrorists learn what can be tapped; they may also learn what cannot be tapped and put that knowledge to use in lethal attacks in the future.

Let us consider what the would-be suicide terrorists and their co-ordinators will be trying to do in the months and years ahead. They will try to hide their affiliations; they will try to plan in secret; they will try to keep their communications secure and will, therefore, try to find out which communications systems cannot be monitored; they will try to find out what MI5 knows about them and their plans; and they will, therefore, try to infiltrate the Security Service to secure all those objectives. Conversely, the Security Service needs to operate in ways in which as little as possible of its intelligence-gathering capacity and its manning and recruitment are revealed.

That is why I am concerned about last weekend's flurry of publicity, and I am grateful to the right hon. Member for Berwick-upon-Tweed (Alan Beith), who I am pleased to see is still in his place, for also voicing his concern. The Home Secretary sought to brush aside my interventions – rather flippantly, I thought – suggesting that the announcements made at the weekend were old reports about a process that had been ongoing for a long time.

It is a fact that all the reports that appeared – I have a sheaf of them from which I could quote at leisure – indicated that recruitment will primarily be for the future. The 1,000 extra surveillance officers will apparently be in place after a time-lapse of three or four more years. I cannot see what could be gained by advertising to people who have an interest in trying to get into the security services for nefarious purposes that the mass recruitment of Arabic speakers has yet to be completed. The matter should have been dealt with in delphic terms, and confirmed only after the process had been carried out.

David Winnick: Will the hon. Gentleman give way?

Dr Lewis: I would rather not, because the hon. Gentleman's hon. Friends will not be able to speak.

I conclude – hopefully in line with my promise to be concise – with two quotes from items in the press today. In the Independent, Dr Brian Jones, formerly the leading nuclear, biological and chemical analyst for the Defence Intelligence Service, who has intimate knowledge of these matters, states:

"From an intelligence and security perspective, it seems rather odd to so publicise a change to an organisation that normally operates with stealth and in the shadows."

Indeed, in the Independent today there is also a small item headed

"Would-be spies rush to visit website".

Underneath we read:

"Visits to MI5's website increased sixfold at the weekend after it was revealed that the security service intended to recruit 1,000 new staff to counter a feared al-Qa'ida attack on Britain.".

I can only hope that the Security Service knows what it is doing in recruiting en masse in this very transparent way. It is a fact that would-be spies are rushing to apply. The question is: are they would-be spies on behalf of MI5 or are they would-be spies against MI5?