The future of President Bush's controversial military trial system for terror suspects held at Guantanamo Bay has been dealt a potentially terminal blow by the US Supreme Court.

In its third rebuke of the Bush Administration's treatment of prisoners at Guantanamo Bay, the court ruled that the 270 foreign terror suspects have the right under the US Constitution to challenge their detention in civilian courts on the American mainland.

The 5-4 ruling did not order the military tribunal process to be halted but it could trigger a chaotic rush to civilian courts that in practical terms will leave the question of what to do with men such as Khalid Sheikh Mohammed, the September 11 mastermind, in the hands of the next president.

Barack Obama, the Democratic nominee, has pledged to close down the site and opposes the military tribunals. John McCain, his Republican opponent, also wants Guantanamo Bay closed. Unlike Mr Obama, however, the Arizona senator supported a law rushed through Congress in 2006 by Mr Bush to resurrect the tribunal system after the Supreme Court last ruled it unconstitutional.

That law, the Military Commissions Act, was passed when Republicans controlled the House and Senate and was the legislation declared unconstitutional by the Supreme Court today, because it denies the detainees the right of habeus corpus - an ability to ask a court if one is being held illegally.

The facility currently has 270 detainees - many of whom have been held without charge for over 6 years.

Democrats are now in the supremacy in both chambers and Mr Bush will be hard pressed to get Congress to circumvent the Supreme Court again.

Five alleged plotters of the September 11 attacks, including Mohammed, appeared in a Guantanamo courtroom last week for a pre-trial hearing, with prosecutors hoping to start a trial on September 15.

Nearly seven years after Mr Bush set up the tribunals, where he argued the detainees had no rights, not one trial has taken place. A military judge at Guantanamo postponed the trial of Osama bin Laden's one-time driver - which had been due to start last week - pending yesterday's Supreme Court ruling.