The Liberal Democrats will highlight the plight of British detainees still held in Guantanamo Bay prison camp with a special debate in parliament.
Ed Davey MP has called for a House of Commons debate to demand action from the government on the ten or more remaining British inmates of the US camp in Cuba, which will mark its fifth anniversary in operation this week.
One of those held is Bisher al-Rawi, a constituent of Mr Davey's who was seized over four years ago but has been held in the camp since then without charge or trial.
"I believe the British government through MI5 has been complicit in his arrest, complicit in his illegal rendition, complicit in his continuing detention and have failed with their moral responsibility to see his early release," Mr Davey told Today.
"So I'll be asking the prime minister today to intervene with President Bush to seek the immediate release of Bisher al-Rawi, and my colleague Sarah Teather will be raising the case of her constituent Jamil el Banna."
He said that according to their lawyers, the mental and physical health of these two men was "deteriorating very fast", adding: "So if the British government doesn't get on and get their release, there won't be much of these men left to release."
Tony Blair has repeatedly said the prison camp at Guantanamo is an "anomaly" that should end, but has refused to go further in joining the condemnation of human rights groups around the world.
His ministers have been speaking out in recent months - in August, constitutional affairs secretary Lord Falconer said the US was guilty of a "shocking affront to the principles of democracy" in using the camp to hold terror suspects.
And in October, foreign secretary Margaret Beckett said the continuing detention of prisoners there without access to a fair trial - most face military tribunal - was "unacceptable in terms of human rights" and "ineffective in terms of counter-terrorism".
But Mr Davey said this morning: "The British government, although it comes on programmes like yours and says it really hates Guantanamo, it really should close, has done very little about it.
"They've not really tackled the American government, both about individual detainees and about the principles behind it."
About 25 MPs have signed an early day motion put by Ms Teather, the Lib Dem education spokesman, to urge ministers to accept their "responsibility and moral duty" towards British detainees at Guantanamo.