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 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 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 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."