I think the jury is out on polygraphs ... we believe it could have benefits

Tom King Intelligence and Security Committee

Sir Stephen told delegates that he welcomed the involvement of politicians in the work of MI5.

But he admitted that the setting up of a parliamentary committee to oversee the work of the security services had made his job harder and had driven up administration costs.

He said that MI5 now employed seven lawyers, instead of one in the past, and a host of accountants.

The Intelligence and Security Committee was set up in 1984 to oversee the administration and policy of the security services.

Recruit vetting

Tom King, chairman of the committee, said that it was actively looking at the possibility of using lie detectors to vet recruits and root out traitors.

Defecting traitors such as Kim Philby once caught the public's imagination

The defections to the then Soviet Union of Guy Burgess, Donald Maclean and Kim Philby, who had all worked for British intelligence, caught the public's imagination in the 1950s and 1960s.

But, in US intelligence at least, the danger and detection of traitors selling secrets to foreign powers is still an active issue even after the end of the Cold War.

"It is important to look at the issue of betrayal," Mr King said.

"I think the jury is out on polygraphs. We believe it could have benefits."

A senior security source told the Press Association that MI5 officers were regularly visiting the US to check on the latest lie detector techniques.

"I don't think the question of polygraphs is a dead question, I think it's an open question," the source said.

'Curious' politicians

Speaking to Thursday's conference, Sir Stephen said: "In the last decade there has been very considerable change for security agencies, bigger than at the end of the Cold War.

"The degree of engagement of ministers in our business has grown exponentially."

The security chief said that as well as inquiring about policy and administrative matters, politicians were becoming more and more curious about operational issues.

"If I had a pound for every time the committee has asked about operational matters I would be a rich man, for example Sierra Leone and Irish terrorism."

Sir Stephen said he was expected to contact the committee to alert them of possible newspaper headlines or if "we have lost another laptop".

"I think the committee has done a good job," he added.