Discussions about the other four prisoners were continuing, he told a news conference.
Once the five return to Britain, Straw said, police would consider whether to arrest them under the Terrorism Act. "Once the detainees are back in the UK, I understand that the police will consider whether to arrest them under the Terrorism Act 2000 for questioning in connection with possible terrorist activity," he said.
Earlier Thursday, Danish Prime Minister Anders Fogh Rasmussen told his parliament that a Dane held at Guantanamo Bay will soon be released.
Danish media have identified him as Slimane Hadj Abderrahmane. He was transferred to the U.S. Naval base in Cuba in February 2001 after being captured in Afghanistan.
"Under Danish law it is not possible to put him on trial. He will come to Denmark as a free man," Foreign Minister Per Stig Moeller said during a debate in parliament.
Five other Guantanamo prisoners — a Spaniard and four Saudi Arabians — were recently released and returned to their countries for detention or possible prosecution there.
Britain has been pressing for months for assurances that the nine men would either face trials that comply with international standards, or be repatriated.
Regarding the Britons who will remain in Cuba, Straw said: "There are a range of security and other issues which we and the American need to consider in respect of these four men."
Moazzam Begg and Feroz Abbasi, who had been listed among the first Guantanamo detainees likely to face trial, were among the four who would not be returned immediately, Straw said. The other two were identified as Richard Belmar and Martin Mubanga.
He insisted that those four Britons would be tried "in accordance with international standards or returned to the UK."
Negotiations about the British detainees have ground along for months. Prime Minister Tony Blair had said in October that he expected the issue to be settled within weeks.
Asked about the delay, Straw said: "It was an entirely unique situation."
He said authorities had obtained useful information from the British detainees. "Valuable intelligence has indeed been obtained and it has helped to make the world a safer place," Straw said.
The five to return were identified as Rhuhel Ahmed, Tarek Dergoul, Jamal al-Harith, Asif Iqbal and Shafiq Rasul.
"There have been many complex issues of law and security which both government have had to consider," Straw said.
"Although significant progress has been made, in the attorney general's view the military commissions as presently constituted would not provide the type of process which we would afford British nationals. Our discussions are continuing," he said.
An American official in January said the United States would allow the British detainees to return home only if they were prevented from engaging in terrorist activity.
Pierre-Richard Prosper, the U.S. ambassador-at-large for war crime issues, said then the nine Britons being held would have to be "detained and investigated, and-or prosecuted" if they came back to Britain.
Prosper said the Guantanamo detainees fell into three categories: Those perceived as the most serious threat, those who posed a medium threat and those who posed no threat or a low threat. He said the Britons fell into the first two categories.
The attorney for one of the men, Louise Christian, said that shouldn't be an issue.
"The idea that the detainees could be a security risk is patently absurd," she told The Times of London. "They could be detained for seven days under the Terrorism Act and questioned by police in the presence of a lawyer.
Some 660 detainees from 44 countries are being held at the base in eastern Cuba on suspicion of links to the fallen Taliban regime of Afghanistan or al Qaeda terror network. U.S. officials decline to provide a breakdown of their citizenship, ages or the reasons they are being held.