It is understood MI5 and MI6 have not been involved in holding or questioning so-called "ghost prisoners".

The Council of Europe watchdog on human rights is looking into reports that the CIA has been using European airports to transport these detainees.

And the EU is also asking the US for clarification on the issue.

Thwarted attacks?

BBC News understands that both MI5 and MI6 are not aware of the locations of any of the CIA's secret prisons, nor the terms of detention for so-called ghost detainees.

British intelligence officers are also said to have had no direct access to the detainees.

But it has been confirmed that the UK has received intelligence based on the interrogations of "ghost prisoners".

Some of the information is said to have led to alleged terrorist attacks in the UK or against UK interests being thwarted.

Interrogation technique

Senior al-Qaeda figures such as Khalid Sheikh Mohammed is among those being held.

He is reported to have been subjected to an interrogation technique called water boarding, in which the detainee is led to believe they are being drowned.

Critics have argued that such practices constitute torture, although the CIA denies this.

But the director of MI5 has recently argued the UK cannot afford to ignore intelligence which may be of value, even if it may be the result of torture.

Investigation

The Council of Europe on Wednesday gave European governments three months to reveal anything they know about any secret prisons or flights involving "ghost" detainees.

The council asked governments to explain "the manner in which their internal law ensures that acts by officials of foreign agencies within their jurisdiction are subject to adequate controls".

Council secretary general Terry Davis, a former British MP, said: "Considering the very serious nature of the allegations, Europe's response should go beyond political statements and inquiries."

He added: "My objective is to find out what, if anything, happened, how it happened and who was involved."

The UK, acting in its role of EU president, said on Tuesday it was seeking clarification from the US on allegations that the CIA has been operating secret prisons in Eastern Europe.

A spokesman for the British Embassy in Washington said the claims remained allegations but it was right to ask the US for more clarification.

The US so far has consistently refused to confirm or deny the reports.

The UK also notes that Romania and Poland - where it was alleged such prisons were based - have already denied they exist.