Lord Goldsmith said he received the assurances during two days of talks with U.S. officials in Washington. He said he had also made significant progress on improving the rules by which British nationals Feroz Abbasi or Moazzam Begg would be tried by a military tribunal.
The men will be able to choose a U.S. civilian lawyer instead of being assigned a military lawyer, he said, and their trials, "subject to any necessary security restrictions," would be open, with news media present.
In a statement in Washington, obtained by the British news agency Press Association, Goldsmith said his objective had been "to ensure that the British detainees in Guantanamo Bay, if prosecuted, are assured of fair trials that meet generally recognized principles, wherever those trials take place, and to make clear our opposition to the death penalty."
Goldsmith met with Defense Department officials on the future of nine British suspects held at Guantanamo.
Abbasi, 23, and Begg, 35, were on President Bush's initial list of six detainees who could face military tribunals at the American base.
"The U.S. has assured us that the prosecution will not seek the death penalty in the cases of Feroz Abbasi and Moazzem Begg," Goldsmith said.
Guantanamo detainees are accused of links to Afghanistan's fallen Taliban regime or the al Qaeda terror network.
Begg has been there for nearly five months and was previously detained in Afghanistan for a year, according to the London-based group Fair Trials Abroad. It said he was seized in Pakistan and may be a victim of mistaken identity.
Abbasi, in U.S. custody since January 2002, has been described as a computer student.
Last week, in a nod to visiting British Prime Minister Tony Blair, the U.S. agreed to suspend legal proceedings against the two suspects until U.S. and British officials could discuss their cases.
Britain is opposed to the death penalty and the Blair government said it would raise the strongest possible objections to any chance of capital punishment being applied in the Britons' cases.
At a joint White House news conference last Thursday, Mr. Bush responded to a reporter's question on the issue, saying, "the only thing we know for certain is that these are bad people and we look forward to working with the Blair government to deal with the issue."
When pressed that such an assertion would do little to assuage British concerns that the men should be viewed as innocent until proven guilty, the president responded, "Let me just say, these were illegal combatants. They were picked up off the battlefield aiding and abetting the Taliban."
The president designated the six suspects as eligible for a military tribunal on July 3. After prosecutors draft charges against the men, it will be up to Deputy Defense Secretary Paul Wolfowitz to decide whether to go ahead with tribunals.
The Pentagon has refused to name the men, state what they were suspected, where they were captured or presently detained, or where they might be tried. It also has not said what would happen if the men were found innocent, raising at least the possibility that they might still be detained.