Foreign Secretary William Hague is facing calls to investigate whether MI6 officers briefed journalists on the rendition of a Libyan opposition activist in breach of the Official Secrets Act.
Tory MP Andrew Tyrie, who chairs the all-party parliamentary group on extraordinary rendition, said recent press reports on the alleged involvement of Labour former foreign secretary Jack Straw appeared to represent a breach by MI6 officers of their duty of confidentiality.
The case relates to the rendition in 2004 of Abdel Hakim Belhadj and his wife from Bangkok to Libya where they say they were tortured by Colonel Gaddafi's security forces. After papers were discovered last year in Tripoli, following the fall of the regime, suggesting MI6 played an active role in the rendition, Mr Straw - who had been foreign secretary at the time - denied authorising any such operation.
However, according to press reports last month, MI6 officers were said to have presented evidence to Mr Straw demonstrating that he had approved Mr Belhadj's rendition.
In a letter to Mr Hague, Mr Tyrie said: "These reports are attributed to 'well-placed sources'. The context suggests that these sources may well be from within MI6.
"If so this would represent staff from the intelligence agencies briefing the press. Such a briefing would be in breach of the Official Secrets Act; as you know, all MI6 employees are required to sign an acknowledgement of their obligations under the Act. As the minister to whom MI6 reports, you will no doubt wish to establish whether such a briefing took place and, if so, take the appropriate disciplinary action."
Mr Belhadj has now launched legal proceedings against Mr Straw and the British Government and his allegations are also the subject of an investigation by Scotland Yard.
Mr Tyrie said the apparent willingness of MI6 officers to brief on this case was at odds with the Government's plans to expand secret court hearings where intelligence material is involved.
"We are constantly told that intelligence material must be protected in the interests of national security, and this argument continues to be used to prevent information relevant to Britain's alleged role in extraordinary rendition from being brought into the public domain," he said.
"If such provisions had been in place it would have been much more difficult, if not impossible, to get the degree of disclosure we have had so far on rendition. The Government needs to re-examine its proposals."