The Liberal Democrat leader, Charles Kennedy, is backing a new legal bid to release four Britons being held as suspected al-Qa'ida terrorists at Guantanamo Bay.
Lawyers for the men will this week ask senior judges to hold a High Court hearing into the Government's perceived failure to get the four men returned to the UK.
In a highly unusual move, their court application has been backed by Mr Kennedy and one of his most senior frontbenchers, Sir Menzies Campbell, the party's foreign affairs spokesman and a QC.
They have filed witness statements stating that President George Bush told them, in a private meeting during his state visit to Britain last year, that if Mr Blair asked for the British detainees to be repatriated "they would be on the first plane home".
Since then, five Britons have been released but four others - Feroz Abbasi, Martin Mbanga, Richard Belmont and Moazzam Begg - are still imprisoned in Camp Delta, more than two years after being detained.
Sir Menzies said yesterday: "We've consistently argued that unless they can be guaranteed a fair trial, they should be returned home. Tony Blair has himself said this on many occasions. He can hardly depart from that position now - particularly in light of the fresh allegations that British detainees at Guantanamo may have been mistreated."
Mr Abbasi, from east London, and Mr Begg, from Manchester, were among the first six detainees to be designated for prosecution by a US military tribunal last July. Their trials were suspended after Britain angrily protested that the tribunals were unfair.
US lawyers acting for the remaining Britons claim they will be held in Cuba until after this November's presidential elections but then released, to avoid any claim before the election that President Bush has gone soft on terrorism.
However, fears that the detainees could be held in Cuba for many years have resurfaced after the Pentagon recently completed a permanent, $31m jail block at Guantanamo Bay called Camp 5, which has been built to house 100 detainees.
Louise Christian, the British lawyer who is taking the case to the High Court this week, said: "The fact that evidence has now emerged of abuse, inhumane conditions and torture means the duty of the British government towards its citizens must be at the forefront."