Lawyers claim secret Government documents show Tony Blair knew as early as January 2002 of allegations of torture involving UK nationals held in Afghanistan and Guantanamo Bay.
A letter from the Foreign Office to the Prime Minister's Office, publicly revealed for the first time today, carries handwritten comments that appear to come from Mr Blair, say solicitors acting for former Guantanamo detainees.
The US detention centre for terror suspects in Cuba was opened on January 11 2002, just days before the letter was sent on January 18.
Six detainees, including Bisher Al Rawi and Binyam Mohammed, are suing the Government over its alleged complicity in their ill treatment.
The letter came to light following a High Court judge's order last July that any documents held by Government departments and the security services on the issue should be disclosed for the legal proceedings, unless disclosure endangered national security.
The letter is entitled "UK Nationals Held in Afghanistan and Guantanamo" and has one whole paragraph redacted.
It was sent by the Foreign Office in response to a request from Downing Street for "a round-up of the latest position". It included a chart listing detainees who might be British.
The person to whom the letter is addressed is blanked out in the released copy, but lawyers for the former detainees say a hand-written comment at the bottom of the letter appears to come from Mr Blair.
The comment reads: "The key is to find out how they are being treated.
"Though I was initially sceptical about claims of torture, we must make it clear to the US that any such action would be totally unacceptable and very quickly establish it isn't happening."
The six men claiming compensation - Binyam Mohamed, Bisher Al Rawi, Jamil El Banna, Richard Belmar, Omar Deghayes, and Martin Mubanga - deny any involvement in terrorism and allege that MI5 and MI6 were guilty of aiding and abetting their unlawful imprisonment and extraordinary rendition to various locations around the world, including Guantanamo, where they were subjected to inhuman and degrading treatment and torture.
Their damages claims are contested by the intelligence services and by the Attorney General, the Foreign Office and the Home Office.
Today lawyers for the former detainees asked Mr Justice Silber, sitting at the High Court in London, to ensure that further secret documents be disclosed to the court without delay in accordance with the disclosure orders he had made in July.
Richard Hermer QC said there had also been a "deeply troubling" failure by the security services and Government departments to answer questions that went to the heart of the case.
"A crucial issue is when the defendants first had knowledge that the Americans might be detaining and subjecting prisoners to inhuman and degrading treatment and torture," said Mr Hermer.
He said dates had still not been given, and asked the judge not to allow the security services any "wriggle-room".
He argued the defendants would use "any conceivably possible excuse" not to comply with the court order.
Jonathan Crow QC, appearing for security services, said it was difficult and time-consuming to get the documents collated and then vetted to ensure no details that could harm national security were not released.
The hearing continues tomorrow.