“The existence of Guantanamo Bay remains unacceptable,” he said. “It is time, in my view, that it should close.”

His speech, to a global security conference at the Royal United Services Institute in London, followed signs that the US wants to reduce the number of alleged militants it holds at Guantanamo, its naval base on the southeastern shore of Cuba.

Lord Goldsmith’s remarks represent a toughening of Britain’s position on Guantanamo. They show that the US’s closest ally in the War on Terror now wants to separate itself from this singularly controversial US policy.

“Not only would it, in my personal opinion, be right to close Guantanamo as a matter of principle, I believe it would also help to remove what has become a symbol to many — right or wrong — of injustice,” he said. “The historic tradition of the US as a beacon of freedom, liberty and of justice deserves the removal of this symbol.”

Tony Blair has called Guantanamo an “anomaly” that would eventually have to be resolved, and Peter Hain, the Northern Ireland Secretary, has said that in his personal view it should close.

There have been recent hints by officials that Britain might even agree to take back captives who were previously long-term British residents, even though they were not British citizens, if that were part of a broader US drive to shut Guantanamo. At least eight captives claim to have been British residents.

But such a move would be hugely sensitive against the backdrop of the row about the release of foreign criminals.

The Attorney-General’s speech steps up the pressure on the US, which has shrugged off criticism by other European governments and the United Nations for breaches of human rights and mistreatment of prisoners since Guantanamo opened in December 2001.

President Bush will no doubt also shrug off the gibe about American hypocrisy over Guantanamo in the letter last weekend from Mahmoud Ahmedinejad, the President of Iran, but it shows the ammunition that the camp is giving to US critics.

More than 750 prisoners have passed through the camp, and just over 490 remain, although the US has so far charged only 10 with crimes. It holds the rest as “enemy combatants” and denies that they are conventional prisoners of war, entitled to the protection of the Geneva Conventions.

The Supreme Court is due to rule by the end of June on the legitimacy of the special military courts in which the US wants to try the accused, and the legality of indefinitely detaining those not charged.

Lord Goldsmith said that “we in the UK were unable to accept that the US military tribunals proposed for those detained at Guantanamo Bay offered sufficient guarantees of a fair trial in accordance with international standards”.

That was a principle “on which there can be no compromise”, he said. After the failure of Washington and London to agree on the trials, the nine British citizens at Guantanamo were returned to Britain, five in 2004 and four last year, where they have been released without charge.

The past month has brought new signs that the US may try to shrink the size of the camp — and the controversy. The US Department of Defence announced that it had listed 141 “detainees” for release or transfer back to their own countries.

This week, in a spectacle that has aroused international bemusement, the US sent five Uighurs — Chinese Muslims from Xinjiang province — from Guantanamo to Albania. It had decided months ago that the Uighurs were not enemy combatants, but could find no country to take them. It feared that if they were sent back to China, which says they are separatist terrorists, they would be tortured or killed.

China this week demanded the Uighurs’ immediate return. Albania, which let them go once to a restaurant with their lawyers, has now penned them up again while it works out what to do. It wants to become a member of the North Atlantic Treaty Organisation — but also to stay friends with China.