Secret Government documents reportedly suggest Tony Blair ordered diplomats to prevent the return to Britain of a UK national detained as a terror suspect and later sent to Guantanamo Bay.
The papers, released as part of a court case being brought by former Guantanamo detainees, show it was decided early in 2002 that the transfer of UK nationals to the US military camp in Cuba was "the best way" to meet the Government's counter-terrorism goals.
And they reveal concern within the Foreign Office that Martin Mubanga's right to consular access was breached as a result of direct interference from 10 Downing Street and that the Government would be "open to charges of concealed extradition".
Another document, published in The Guardian, appears to record an interrogation attended by UK agents at Bagram air base in Afghanistan where detainee Omar Deghayes complained of being treated in a "barbaric" fashion by his American captors using leg braces and "lock-down positions".
The papers were among many classified documents disclosed this week as part of a case being pursued at the High Court by six former Guantanamo detainees against the security and intelligence services and Government departments. Large portions of text were blacked out by Government lawyers before their release.
Prime Minister David Cameron made an offer of mediation and eventual compensation to the men in order to bring legal proceedings to a close and clear the way for an inquiry into allegations that Britain was complicit in torture.
The documents obtained by legal action charity Reprieve mainly revolve around the period following the toppling of the Taliban regime in Afghanistan in 2001, when many suspects were detained in connection with alleged involvement in the al Qaida terror network.
A Foreign Office telegram in January 2002 stated that the UK position on the repatriation and prosecution of British nationals among the suspects was "still being considered". But it added: "We accept that the transfer of UK nationals held by US forces in Afghanistan to the US base in Guantanamo is the best way to meet our counter-terrorism objective by ensuring that they are securely held."
A later exchange of messages about Mr Mubanga, who held joint UK and Zambian nationality and was detained in the African country, revealed that diplomats felt they were unable to offer him consular assistance as "our hands were tied by policy directed from London".
An unnamed Foreign Office official stated: "Instructions from London were unequivocal. We should not accept responsibility for or take custody of (Mubanga). This was subsequently reinforced by the message from No 10 that under no circumstances should Mubanga be allowed to return to the UK. It became clear that if we requested consular access... thereby de facto acknowledging him as a UK national, he would have been handed over to us. This would have gone against all other instructions from London."