Publication of guidance for Britain's intelligence agents detailing what counts as complicity in torture should not be withheld on grounds of national security, Ed Davey has said.
The Liberal Democrats' foreign affairs spokesman told politics.co.uk such arguments were "absolute rubbish" as he supported Reprieve's legal proceedings against the government on the guidance.
Prime minister Gordon Brown told the Commons he intended to publish revised guidance when it was ready in March last year, but is refusing to release the guidance in use from 2004 to 2010.
"There are now so many allegations of British complicity in torture we feel they need to be looked at," he said.
We also feel it's in the interests of Britain's national security and the reputation of our secret services, because increasingly it looks as if the policy guidance which would have been signed off by ministers may be at fault here.
"That our intelligent agents were working under a policy framework set by ministers which itself was illegal. If that's the case, that's very serious."
The Lib Dems have called for a judicial inquiry into the claims, echoing the demands of former attorney general Lord Goldsmith and the Equality and Human Rights Commission chair Trevor Phillips.
Mr Davey said the government's "stubborn refusal to allow a light to be shined on these allegations" meant Reprive's approach was "probably the only way forward".
Foreign secretary David Miliband has insisted that the 2004 guidance should be kept from the public even if ordered to be disclosed in court proceedings, according to Reprieve.
Today a Cabinet Office spokesman said: "It has been the longstanding practice of the government not to comment upon operational matters or to place operational material into the public domain.
"Recognising the importance of this particular issue, however, the prime minister has made an unprecedented commitment to publish consolidated guidance in order to make plain the standards to which we hold ourselves.
"The decision to publish the consolidated guidance is therefore very much the exception to the rule and we have no intention to publish the previous material, or to change our general practice in this area."
Mr Davey said ministers' preparedness to publish the new guidance undermined its claims that the old guidance was not politically embarrassing.
"If the government believes it was lawful there's no reason why it shouldn't be put in the public domain," he said.
"It's not about individual operational cases - it's about the overall policy framework. The fact they're not prepared to put it in the public domain smells very dodgy to me."