The British attorney general, Lord Goldsmith, is set to trigger a diplomatic row between London and Washington by calling for the closure of the detention facility at Guantanamo Bay.
The decision by the UK government's chief legal adviser to denounce the detention center in Cuba as "unacceptable" will dismay the administration of US President George W. Bush which has continually rejected claims that the camp breaches international laws on human rights.
But Goldsmith will tell a global security conference at the Royal United Services Institute in London this week that the camp at Guantanamo Bay must not continue.
An urbane lawyer who eschews the limelight it is clear that Goldsmith has harbored grave doubts for some time over the legality of Guantanamo under international law.
"There are certain principles on which there can be no compromise," Goldsmith will say. "Fair trial is one of those -- which is the reason 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."
Although privately some senior British ministers believe Guantanamo should be closed down, no one has so far condemned the camp in such open and trenchant terms.
To date, the strongest criticism of the camp has come from Peter Hain, the Northern Ireland minister, who said on BBC TV's current affairs show, Newsnight, in February that it was his personal belief that the camp should close, while the prime minister said only that it is an "anomaly" that will have to end one day.
Goldsmith's speech will be welcomed by human-rights groups and senior members of the judiciary who have long campaigned for the government to use its influence to persuade its ally to close the camp.