The UK government has requested the release of five British residents from US custody at Guantanamo Bay.
The men are being detained at Guantanamo Bay
The men are not British citizens but lived in the UK before they were detained by the US.
The request is a change of policy for the government which had previously said it could not intercede for non-British citizens.
Foreign Secretary David Miliband formally wrote to his US counterpart Condoleezza Rice with the request.
The five men are Shaker Abdur-Raheem Aamer, Jamil el-Banna, Omar Deghayes, Binyam Mohammed al Habashi and Abdulnour Sameur.
They were former UK residents who had either been granted refugee status, indefinite leave or exceptional leave to remain prior to their detention.
The Foreign Office said "discussions with the US government about the release and return of these five men may take some time".
Omar Deghayes, pictured above, Libyan with refugee status
Shaker Abdur-Raheem Aamer, Saudi Arabian granted indefinite leave to remain
Jamil el-Banna, Jordanian with refugee status
Binyam Mohammed al Habashi, Ethiopian asylum seeker
Abdulnour Sameur, Algerian with leave to remain
"The government will of course continue to take all necessary measures to maintain national security.
"Should these men be returned to the UK, the same security considerations and actions will apply to them as would apply to any other foreign national in this country."
The Foreign Secretary and Home Secretary said they had made the request because of recent steps taken by the US government to reduce the number of inmates at the camp in Cuba and "to move towards the closure of the detention facility".
"These steps include an increasing emphasis on engagement with third countries over the transfer and resettlement of those detained," the Foreign Office said.
US ambassador Robert Tuttle said the US "has no desire to be the world's jailor" and wanted to close Guantanamo Bay by moving people to countries which would treat them humanely.
"We will take the request to release them and study it very seriously and get back with all due, deliberate speed," he told BBC Radio 4's The World at One.
BBC world affairs correspondent Paul Reynolds said that while the British government had sought to present the decision as the result of a greater American willingness to engage with third-country governments it was a clear change of UK policy.
The new British government under Gordon Brown seems willing to be more flexible in its conduct of the "war on terror" declared by President Bush, our correspondent added.
The government said all UK nationals had been released from Guantanamo Bay by January 2005.
But it had refused to act on behalf of the five men - a decision upheld by the Court of Appeal last year which agreed that requesting the return of non-British nationals would be counterproductive as the US had clearly said it would not negotiate with third countries.
"We judged that it would also have been counter-productive, at that time, to our wider aim of securing closure of the detention facility," the FCO said.
"The situation has now changed and the Foreign Secretary and Home Secretary have reviewed the government's approach in light of these circumstances."
Lib Dem frontbencher Sarah Teather, who is Mr Banna's MP in Brent East, said she and his family were "overwhelmingly happy" at the news, especially as his youngest daughter had never met her father.
But she said she was disappointed that the government had taken four years to make the request.
She said he had already been cleared for release by the US.
"He's jumped through all the hoops to prove that he is no threat whatsoever. I'm sure the British government will do whatever is required if they believe [the men] to be a threat," she told BBC News 24.
"If they had done something wrong they should have been put on trial, they should have had an opportunity to demonstrate their innocence or for the other side to prove their guilt and at no stage has that happened."
Human rights group Liberty welcomed the UK government's request. Legal director James Welch said: "This change of policy is extremely welcome, especially if it signals a bigger change of approach on both sides of the Atlantic.
"Surely US and UK governments need no further evidence that internment, kidnap and torture have been completely counterproductive in the struggle against terrorism.
"It's high time that the special relationship returned to its original values of defending liberty rather than degrading it."