Andrew Tyrie: How many more times will they change the story on torture?

Six years ago, I started to ask questions about rendition and got no proper answers. I have since made specific allegations: that the UK has facilitated rendition; that Diego Garcia was used for this purpose; that UK armed forces were dragged into rendition. All were met with blanket denials. The then foreign secretary, Jack Straw, likened such allegations to "conspiracy theories". Each has now been admitted.

Last week's Court of Appeal judgment, reinstating a paragraph of its judgment in the Binyam Mohamed case, appears to take UK involvement further. The judgment is highly critical of the security services' behaviour, suggesting that Parliament's Intelligence and Security Committee (ISC) and then foreign secretary may have been misled, devaluing the assurances they have both given.

The High Court had already found that the UK's role in the rendition of Mohamed "was far beyond that of a bystander or witness to the alleged wrongdoing", and that the UK had "facilitated" his interrogation by the US. In last week's judgement, the Master of the Rolls was critical of the officer who interrogated Mohamed, known as Witness B, whose involvement was "dubious" and who was less than "frank" about his involvement. The guidelines under which Witness B was operating should be published immediately. It is important to establish if he was acting on his own initiative, or whether his activities were authorised by senior MI5 officers.

I have made simultaneous Freedom of Information requests in both the UK and the US for records of British involvement. They will decide if government reluctance to provide this information is justified, or if the information should be published.

The extent to which the ISC appears to have been misled reinforces concerns about its ability to do its job. It is composed of senior colleagues trying to do their best but they face at least two obstacles. First, the committee has been without an investigator since John Morrison was sacked in 2004. This role must be filled. Second, it does not appear sufficiently independent of the Prime Minister. Reform of the way the chairman is appointed is essential. The Wright Committee's recommendation that the chairman be elected by MPs, subject to a prime ministerial veto, can bolster accountability, and should be implemented by Parliament when it is considered on Thursday.

With or without the ISC, the truth will out, eventually. But the current drip-drip of information on UK involvement in rendition is corrosive, saps the morale of our intelligence services, and are probably counter-productive for intelligence gathering. Furthermore, the services want the public to have confidence in them. Accountability is to their benefit. That is why we do them a disservice if we fail to investigate credible allegations. The quickest and most effective way to do this is the "brief, judge-led inquiry" for which David Cameron has called.

Such an inquiry has been challenged on the basis that some of the issues are being investigated by the police or are before the courts. This is a straw man. Inquiries often overlap with police investigations, and sometimes with prosecutions. With Cameron, Lib Dem leader Nick Clegg, Lord Carlile (the Government's own independent reviewer of terrorism legislation), and Lord Goldsmith also calling for an investigation, only the Government is holding out. Its time to get the job done, find out the truth and move on.

Andrew Tyrie is chair of the all-party parliamentary group on extraordinary rendition and Tory MP for Chichester