The Pentagon insists the three men are dangerous
Three British residents held by the US at Guantanamo Bay for four-and-a-half years have been detained after arriving back in the UK.
Omar Deghayes and Abdenour Samuer were arrested under the Terrorism Act after they arrived at Luton Airport and are being questioned at Paddington Green.
Jamil el-Banna was not arrested but is being detained under the act and questioned at a Luton police station.
The BBC understands two of the men may face an extradition request from Spain.
BBC home affairs correspondent Daniel Sandford said Mr el-Banna would face such a request while Mr Deghayes could also be wanted by the Spanish authorities.
The men's lawyer, Clive Stafford-Smith, told BBC's Newsnight that the Spanish case was based on false evidence.
He said he was "dismayed" at the situation with Spain and would fight any attempt to extradite them.
"The fact that the Spanish actually were behind this wrongful detention in Guantanamo Bay is something they should be ashamed of," he said.
"The idea now that they want to use this evidence we've proved to be false to take them for further detention is very worrying."
The government has said the three men's immigration status will be reviewed.
Another freed UK resident, Shaker Abdur-Raheem Aamer, is expected to return to his native Saudi Arabia.
A fifth UK resident, Ethiopian Binyam Mohammed, will remain at Guantanamo.
A Home Office statement said the US had agreed to the release of the three men on 10 December.
"This does not imply a commitment on our part that they can remain permanently in the UK and their immigration status will be reviewed immediately following their return," the statement added.
Earlier, Mr Stafford-Smith said: "All of our clients have voluntarily agreed to voluntary security arrangements requested by the British government.
"I can't talk about the details because that's what the British government asked.
"We agreed to exactly what the British government wanted because our clients have absolutely nothing to hide."
He said one of the men, Mr Deghayes, had been blinded in his right eye as a result of "US mistreatment" at Guantanamo.
A Metropolitan Police spokesman said: "All the men will be medically examined by a forensic medical examiner to ensure that they are fit to be detained and interviewed by police."
Mr Deghayes' sister, Amani, said his family would be concentrating on helping him to put his ordeal behind him.
"I'm extremely relieved that Omar's ordeal is finally coming to end after over five years of suffering in Guantanamo," she said.
Two of the men were taken to Paddington Green for questioning
"We're looking forward to spending the Eid as family together."
She said her brother had been on the receiving end of "brutal and illegal treatment".
Human rights solicitor Gareth Peirce, who has previously represented two of the men, said their families could not wait to see them.
"It happens that today is Eid so it is particularly poignant."
The Americans accuse Palestinian Mr el-Banna of being an al-Qaeda recruiter and financier, Libyan Mr Deghayes of associating with al-Qaeda, and Algerian Mr Sameur of being trained for combat in Afghanistan.
There have been intensive negotiations between the UK and US authorities over the past few months.
The Pentagon insists that all five of the British residents are dangerous.
Meanwhile, five Frenchmen who were detained at Guantanamo have been convicted of having links to terrorism by a court in Paris.
All five were sentenced to one year in jail plus a suspended sentence, but will not return to jail having spent more than a year in US custody.
About 300 prisoners are held at Guantanamo Bay, set up at a US naval base in Cuba after the invasion of Afghanistan in early 2002.
The US argues that foreign nationals captured and detained outside the US have no recognisable constitutional rights.
Amnesty International's UK director, Kate Allen, welcomed the release of the three men and said they should be treated "first and foremost as victims of a serious miscarriage of justice".
"It's important that the government speaks out about the hundreds of men still held there - including at least two other men with ties to Britain - Ahmed Belbacha and Binyam Mohammed. These men must not become Guantanamo's forgotten prisoners."
She called on ministers to condemn the practices of rendition and secret detention, which the organisation claims "have fed the system at Guantanamo in the past six years".
Responding to the release of the men, the Conservatives said the government has failed to give assurances on "issues of national security" posed by the men.
Shadow home secretary David Davis said: "Does the home secretary agree with the US that these men are extremely dangerous?
"If so, how will the government guarantee the safety of the British public when they arrive in the UK?"