The government has backed a police investigation that found no evidence the CIA had used UK airports to transport terrorism suspects illegally.
The Council of Europe has accused the UK of collusion
Ministers said the 18-month inquiry into "extraordinary rendition" tallied with their research into the issue.
Human rights groups have labelled the police report a whitewash, claiming 210 flights had been in the UK since 2001.
A powerful committee of MPs has been investigating the government's role and is expected to produce its report soon.
The Liberal Democrats said the Intelligence and Security Committee was better placed than the police to uncover evidence that flights carrying terrorism suspects for possible torture in other countries had passed through the UK.
'Only way forward'
The party's deputy leader Vince Cable said there was "little doubt" Britain had breached "substantial international law obligations".
He believes European countries and the US reached an agreement over the flights in 2001, in the wake of the 11 September attacks.
"Only a few, very senior British intelligence officers and ministers actually knew what the arrangement was," he said.
"If the police were able to pursue this, they'd have to talk to those people and that's clearly rather challenging.
"I think the only way forward in the short-run is a parliamentary inquiry, which I believe is already under way."
Human rights groups the Council of Europe and Liberty believe the UK and other European countries colluded with the CIA between 2001 and 2005.
Liberty sparked an inquiry by the Association of Chief Police Officers (Acpo) last year when it sent a letter to the authorities detailing claims of CIA flights.
But Acpo said on Friday it had found "no evidence to substantiate Liberty's allegations"
The Foreign Office previously said the US had requested permission to transfer detainees through the UK on four occasions - all of them in 1998.
Two of the requests had been, and the individuals had stood trial in the US, according to the Foreign Office.
The government insists its own searches of records since 1997 have revealed no evidence of people being transported through the UK where there were substantial grounds to believe they might be at risk of torture.