Barack Obama will revive the heavily criticised George Bush-era military tribunals for detainees at Guantánamo Bay but will make them fairer, according to US officials.
Obama suspended the tribunals within hours of taking office in January, ordering a review of the military commission system. But he stopped short of abandoning the process altogether.
The military trials will remain frozen for another four months as the administration adjusts the legal system - expected to try fewer than 20 of the 241 detainees at Guantánamo Bay in Cuba.
Officials say the amended system would limit the use of hearsay and ban evidence gained from cruel treatment, including the use of "waterboarding", a form of simulated drowning.
Defendants will be given the opportunity to pick their own lawyers and be provided with more protection if they do not testify. The decision to persist with the tribunals was immediately attacked by critics.
"It's disappointing that Obama is seeking to revive rather than end this failed experiment," said Jonathan Hafetz, a national security lawyer at the American Civil Liberties Union.
"There's no detainee at Guantánamo who cannot be tried and shouldn't be tried in the regular federal courts system."
Human rights campaigners point out that during last year's presidential campaign Obama called the tribunal system "an enormous failure" and that he vowed to "reject the Military Commissions Act".
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But the president's aides say he never rejected the possibility of using military commissions altogether, if they could be made fairer.
They highlighted legislation he supported as a senator in 2006, intended to do just that, and they noted that some defendants would continue to be tried in American civilian courts, saying they were trying to create a "durable, multi-layered option".
Thirteen detainees, including five charged over the September 11 attacks, are already in the tribunal system, set up after the military began sweeping the battlefields of Afghanistan in late 2001.
The Republican senator Lindsey Graham described Obama's decision to revamp the tribunals as a step forward.
"I agree with the president and our military commanders that now is the time to start over and strengthen our detention policies. I applaud the president's actions today," said Graham, who has been working with the administration on issues related to detainees.
Three Guantánamo detainees have been convicted in the tribunals so far and the restrictions on evidence will almost certainly mean only a fraction of those held will go to trial.
The rest of the detainees would either be released, transferred to other countries or tried in civilian courts, officials said.
The decision to restart the process puts the administration in a race against the clock to conclude commission trials before the navy prison is closed by January 2010.