The UK has helped the US fly terror suspects to secret detention camps, says a report from Europe's leading human rights watchdog.
CIA flights have landed in European countries, Mr Marty says
A Council of Europe inquiry says the CIA used Prestwick Airport in Scotland as a stop-off point. It claims the UK gave details used in alleged torture.
Tony Blair told MPs the report contained "absolutely nothing new".
But minister Harriet Harman suggested the rules on declaring what planes are carrying need to be examined.
The report follows a seven-month inquiry by Swiss MP Dick Marty.
Mr Blair was confronted about the findings at prime minister's questions by Liberal Democrat leader Sir Menzies Campbell.
Sir Menzies asked him to confirm that the UK had given no logistical support to the CIA on the flights - known as renditions - nor given information to be used in torture.
The prime minister said the government had said all it had to say on the issue.
He repeated the government's insistence that since 1998 it had agreed to two US requests for prisoner flights through the UK, and refused two others.
Conservative MP Andrew Tyrie, chairman of the all-party group on extraordinary rendition, said it would not wash for Mr Blair to refuse to say more about the issue.
"They have done their best to put up the shutters and walk on the other side of the street," he said.
Web of flights
The report concludes that a "spider's web" of flights criss-crossed Europe, and included secret jails in Poland and Romania.
Both countries had previously denied the claims and Mr Marty admits there is "no formal evidence" of secret detention centres.
But he says it is clear an unspecified number of people were unlawfully arrested and/or detained and transported by services acting for the US.
Mr Marty specifically criticises the UK for helping in the detention and mistreatment of Binyam Mohamed al Habashi, an Ethiopian citizen who was a UK resident from 1994.
He is now being held at Guantanamo Bay.
Mr Marty points to claims that Mr al Habashi, who says he was arrested in Pakistan after visiting Afghanistan, was tortured in Morocco to try to get him to confess to terrorist activities.
His interrogators, one of whom the report says is thought to have been a CIA agent, used personal information about him to suggest they knew a lot about it.
"Much of the personal information - including details of his education, his friendships in London and even his kickboxing trainer - could only have originated from collusion in this interrogation process by UK intelligence services," says Mr Marty.
"Since the purposes to which this information would be put were reasonably foreseeable, the provision of this information by the British Government amounts to complicity in Binyam's detention and ill-treatment."
The report also slams MI5 for cooperating with the CIA in "abducting persons against which there is no evidence enabling them to be kept in prison lawfully".
It cites the case of Bisher Al-Rawi and Jamil El-Banna, two UK residents who were arrested in Gambia and later transferred to Afghanistan and then Guantanamo Bay.
Constitutional Affairs Minister Ms Harman said the UK would not be complicit in flying prisoners to countries where they might be tortured.
She said it would be "outrageous" if the US had flown prisoners through UK bases or airspace without asking permission.
But US Secretary of State Condoleezza Rice had assured former Foreign Secretary Jack Straw this had not happened, she said.
Ms Harman said aircraft operators currently had to declare if they were carrying dangerous substances or VIPs through foreign airspace, but not prisoners, she said.
"I think actually looking at the international treaties about disclosure is something that needs to be looked at," Ms Harman told BBC Two's Daily Politics.
'Almost as bad as terrorists'
Former Foreign Office Minister Tony Lloyd said attempts to fight terrorism could be damaged by renditions.
"It leads to a suspicion that at very best we are using the wrong tactics or even that what we are doing is almost of itself as bad as the terrorists," he said.
Mike Gapes, chairman of the Commons foreign affairs select committee, said the report contained little new information.
Details from flight logs about alleged CIA planes did not show whether they were being used for rendition, he argued.