The government is secretly trying to stifle attempts by MPs to find out what it knows about CIA "torture flights" and privately admits that people captured by British forces could have been sent illegally to interrogation centres. A hidden strategy aimed at suppressing a debate about rendition - the US practice of transporting detainees to secret centres where they are at risk of being tortured - is revealed in a briefing paper sent by the Foreign Office to No 10.

The document shows that the government has been aware of secret interrogation centres, despite ministers' denials. It admits that the government has no idea whether individuals seized by British troops in Iraq or Afghanistan have been sent to the secret centres.

Dated December 7 last year, the document is a note from Irfan Siddiq, of the foreign secretary's private office, to Grace Cassy in Tony Blair's office. It was obtained by the New Statesman magazine, whose latest issue is published today.

It was drawn up in response to a Downing Street request for advice "on substance and handling" of the controversy over CIA rendition flights and allegations of Britain's connivance in the practice.

"We should try to avoid getting drawn on detail", Mr Siddiq writes, "and to try to move the debate on, in as front foot a way we can, underlining all the time the strong anti-terrorist rationale for close cooperation with the US, within our legal obligations."

The document advises the government to rely on a statement by Condoleezza Rice last month when the US secretary of state said America did not transport anyone to a country where it believed they would be tortured and that, "where appropriate", Washington would seek assurances.

The document notes: "We would not want to cast doubt on the principle of such government-to-government assurances, not least given our own attempts to secure these from countries to which we wish to deport their nationals suspected of involvement in terrorism: Algeria etc."

The document says that in the most common use of the term - namely, involving real risk of torture - rendition could never be legal. It also says that the US emphasised torture but not "cruel, inhuman and degrading treatment", which binds Britain under the European convention on human rights. British courts have adopted a lower threshold of what constitutes torture than the US has.

The note includes questions and answers on a number of issues. "Would cooperating with a US rendition operation be illegal?", it asks, and gives the response: "Where we have no knowledge of illegality, but allegations are brought to our attention, we ought to make reasonable enquiries". It asks: "How do we know whether those our armed forces have helped to capture in Iraq or Afghanistan have subsequently been sent to interrogation centres?" The reply given is: "Cabinet Office is researching this with MoD [Ministry of Defence]. But we understand the basic answer is that we have no mechanism for establishing this, though we would not ourselves question such detainees while they were in such facilities".

Ministers have persistently taken the line, in answers to MPs' questions, that they were unaware of CIA rendition flights passing through Britain or of secret interrogation centres.

On December 7 - the date of the leaked document - Charles Kennedy, then Liberal Democrat leader, asked Mr Blair when he was first made aware of the American rendition flights, and when he approved them. Mr Blair replied: "In respect of airports, I do not know what the right hon gentleman is referring to."

On December 22, asked at his monthly press conference about the US practice of rendition, the prime minister told journalists: "It is not something that I have ever actually come across until this whole thing has blown up, and I don't know anything about it." He said he had never heard of secret interrogation camps in Europe. But Jack Straw, the foreign secretary, recently disclosed that Whitehall inquiries had shown Britain had received rendition requests from the Clinton administration.

In 1998, Mr Straw, then home secretary, agreed to one request, but turned down another because the individual concerned was to be transported to Egypt. He agreed that Mohammed Rashed Daoud al-Owhali, suspected of involvement in the bombing of the US embassy in Nairobi, could be transported to the US for trial via Stansted, according to the briefing paper. Owhali was subsequently given a life sentence.

Shami Chakrabarti, director of the human rights group Liberty, which has demanded an inquiry into allegations of British collusion in rendition flights, said she was "deeply disappointed" by the memo. "The government seems more concerned about spinning than investigating our concerns," she said. She has written to Mr Straw saying the government must now give its full support to the inquiry conducted, at Liberty's behest, by the chief constable of Greater Manchester, Michael Todd.

Nick Clegg, the Liberal Democrats' foreign affairs spokesman, said Mr Blair had fully endorsed Ms Rice's statement, yet the prime minister had clear advice that it might have been deliberately worded to allow for cruel, inhuman and degrading treatment. "I am submitting an urgent question to the speaker and expect the foreign secretary to come to parliament to explain the government's position," he said. "Evasion can no longer be sustained: there is now overwhelming evidence to support a full public inquiry into rendition."

Andrew Tyrie, Conservative MP for Chichester and chairman of the parliamentary group on rendition, said last night: "All the experts who have looked at Rice's assurances have concluded that they are so carefully worded as to be virtually worthless. Relying on them, as the government appears to be doing, speaks volumes". He said his committee would pursue the issue.