Meanwhile, the Danish government has announced one Dane will also be freed soon, but Denmark's media have nevertheless blasted their government and Washington for disregarding international law.
The five British nationals are among nine Britons, and a total of more than 650 prisoners, at the isolated US naval base where the Bush administration locks up non-American suspects in its "war on terror". All nine detainees were captured in Afghanistan in late 2001 and 2002.
British Foreign Secretary Jack Straw announced on Thursday that, after months of negotiations between London and Washington, the five will be flown home to Britain "in the next few weeks". BBC News said they would be told of their impending release later on Friday.
Straw added that talks were continuing with US officials on the fate of the other four Britons.
Home Secretary David Blunkett said on Thursday: "No one who is returned... will actually be a threat to the security of the British people."
But London's Metropolitan Police said it was immediately opening an investigation into the five to see if any might be suspected of violating British anti-terrorist laws.
Most of the prisoners at Guantanamo Bay were captured in Afghanistan as part of the US-led "war on terror" following the 11 September, 2001 attacks in New York and Washington blamed on Usama bin Ladin's al-Qaida group.
The Bush administration has called the detainees "enemy combatants" – a status not recognised by international law – rather than as prisoners of war to hold them indefinitely beyond the reach of US civilian courts and outside the realm of the Geneva Conventions.
The five Britons being released were identified as Rhuhel Ahmed, 23, Asif Iqbal, 20, and Shafiq Rasul, 25, all from Tipton, near Birmingham, Tarek Dergoul, 24, from east London, and Jamal al-Harith, 35, from Manchester.
Amnesty International called the releases arbitrary.
"Considering that none of the five Britons has been charged with any crime or even had access to a lawyer, these releases are long overdue," said spokeswoman Lesley Warner.
"This has the feeling of a lottery: five men are to be transferred to the UK, yet some 650 - including Britons - remain in legal limbo without even being charged, let alone given a fair trial."
Meanwhile, the Australian government refused on Friday to bow to demands by lawyers and civil liberty activists to press the US to release two suspected terrorists because they could not be prosecuted here.
Former kangaroo hunter David Hicks and Egyptian-born Australian Mamduh Habib will remain in detention despite the release of five British citizens and one Dane, Attorney-General Philip Ruddock said.
Australian anti-terrorist laws were not introduced until after the attacks on New York and the Pentagon and are not retrospective.
Adelaide-born Muslim convert Hicks, 28, was captured in November 2001 fighting for the Islamic fundamentalist Taliban against US-backed forces in Afghanistan.
Habib, 48, said to be an outspoken advocate of Islamic causes in Sydney, travelled to Pakistan in September 2001, reportedly to arrange schooling for his children, and was arrested by Pakistani intelligence in Karachi that month.
In Denmark, the press greeted the news that a Dane wold also be released by attacking the government for its passivity in the face of US violation of international law.
"It's a scandal that the governments of 42 countries who have citizens locked up at Guantanamo have been so patient and accommodating about the way the United States has treated their nationals," the daily Politiken said.