PlayForeign Secretary David Miliband has said there are "some concerns" over how the US will try six men accused of the September 11 attacks.
If sent for trial, six Guantanamo Bay detainees would be the first to be brought before a military tribunal at the Cuban base in connection with the 2001 atrocity which killed almost 3,000 people.
If found guilty, the execution of the six, including Khalid Sheik Mohammed, the alleged mastermind of the plot, would then be sought by prosecutors.
Walid bin Attash, Ramzi bin al-Shibh, Ali Abdul Aziz Ali, Mustafa Ahmed al-Hawsawi and Mohammed al-Kahtani are also charged with committing a total of 169 overt acts in connection with the attacks.
Mr Miliband said: "He (Mohammed) needs to be tried properly with a full defence and other legal rights."
Asked whether the tribunal would respect Mohammed's legal rights, the Foreign Secretary replied: "We have some concerns about that."
On Monday, Brigadier General Thomas Hartmann told a Pentagon press briefing that Mohammed had proposed the plan to al-Qaeda leader Osama bin Laden "as early as 1996".
Previously, the Pentagon has said that Mohammed confessed to his role in the attacks, saying: "I was responsible for the 9/11 operation from A to Z."
The confession followed his interrogation by US authorities, reportedly including the controversial technique of waterboarding - or simulated drowning - a form of torture.
Officials plan to hold the trial in a specially constructed court at Guantanamo that will allow lawyers, journalists and some others to be present, but leave victims' relatives and others to watch the trial through closed-circuit broadcasts.
It is likely to be months or longer before the trial begins for the six defendants.
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