Jack Straw's statement that there are no records of the CIA flying terror suspects through the UK to face torture may be "worthless", a group of MPs say.
Ministers are under pressure to investigate further
The MPs are not reassured by the foreign secretary and say they have legal advice suggesting the government must actively investigate the claims.
But Mr Straw later told MPs there was no need at all for a judge-led inquiry.
And he said that unless people believed he and his US counterpart were lying, there was no truth in the allegations.
The foreign secretary was giving evidence to the Commons foreign affairs committee.
He said officials had no records of requests from the US for flights carrying suspects to pass through UK territory and airspace during George Bush's presidency.
Nor had other papers been discovered on the issue.
His comments comes on the day a report to the Council of Europe said claims about the flights were "credible".
Swiss senator Dick Marty told the Council of Europe there were indications that the CIA had abducted and illegally transported terror suspects across European borders.
Conservative MP Andrew Tyrie, who co-founded the all party parliamentary group on extraordinary rendition, said Mr Straw's comments should "reassure nobody".
He said it was "hardly surprising" there were no records because the Home Office had said "records of transit application are not kept once the transit has been completed".
"It is crystal clear that the UK must investigate allegations that it has been complicit in torture," argued Mr Tyrie.
"Checking for instances of the US requesting permission is simply derisory."
The group of MPs asked James Crawford, Whewell Professor of International Law at Cambridge, for legal advice about the government's responsibilities on investigating the issue.
Professor Crawford argues that to comply with its legal obligations, the UK government must satisfy itself that secret CIA flights are not leading to torture.
He said: "The question that must be asked is whether torture is likely to take place if a person is transported, irrespective of whether or not the government claims that the answer is no, or what its hopes or beliefs may be."
He added: "A government is not exonerated from conduct which leads directly to a person being tortured merely by closing its eyes to that prospect."
US Secretary of State Condoleezza Rice has admitted terror suspects are flown abroad for interrogation, but denies they are tortured.
She said suspects were moved by plane, and it was under a process known as rendition which was a "lawful weapon".
The all party group of MPs says ministers can get little or no legal cover by relying on Dr Rice's assurances.
During his grilling by the foreign affairs committee, Mr Straw said it was impossible to "prove a negative" but it was "extremely improbable" that any rendition flights had passed through the UK.
No records of requests from the US, or other papers corroborating the claims had been found, and there were the assurances from Dr Rice that suspects were not being taken to torture.
"Unless we all start to believe in conspiracy theories and that the officials are lying, I'm lying and that behind this there is some kind of secret state in league with some dark forces in the US, and we believe Secretary Rice is lying, there is simply no truth in claims that the UK has been involved in rendition," he said.
Mr Straw also confronted claims that a British intelligence officer had handed a man called Benyam Mohammed al-Habashi to the CIA and he had later been tortured.
The foreign secretary said the British security services had no role in Mr al-Habashi's capture or transfer from Pakistan.
Nor had the security service officer involved observed any abuse, he said.
Mr Straw said he would consider whether he could tell MPs if British services had transferred the man to the CIA inside Pakistan.