(Adds Home Office comment paragraphs 9-10)

By Peter Graff

LONDON, Aug 4 (Reuters) - Britain's highest court reopened on Monday the question of whether authorities can hold foreign terrorism suspects without charge, ordering the government to defend one of its most controversial security practices.

Britain is holding 13 foreign "suspected international terrorists" under a new law that allows them to be jailed indefinitely without charge or trial.

An immigration commission initially ruled the detentions were illegal because the rules applied only to foreigners. But last year an appeals court upheld the government's case, ruling the detentions were allowed.

Three of Britain's "law lords" ruled in a decision published on Monday that the detainees could appeal to the House of Lords, which acts both as Britain's upper house of parliament and its highest court.

The law lords act as judges on behalf of the house.

They gave no reason for the decision to allow the appeal, but lawyers for the detainees hailed it.

"Basically it would have been scandalous if they didn't grant leave (to appeal) because it is a constitutional issue," Natalie Garcia, lawyer for two of the detainees, told Reuters.

"Britain has derogated from article five of the European Convention on Human Rights, which says no one shall be detained without trial," she said. "It's a complete travesty of justice. There's no way that people should be detained on this basis."

A spokeswoman for the government's Home Office (Interior Ministry), which is responsible for the cases, said the government stood firmly behind the anti-terror laws.

"We maintain the view that, whilst the public emergency continues to exist, these provisions are both necessary and proportionate, and consistent with our international obligations," she said.

The government says that under the law, it has to demonstrate only that it has "reasonable grounds to suspect" that detainees have links to terrorism -- it does not actually have to charge them with a crime or prove their guilt.

Hearings by a special immigration commission have been under way for weeks in the cases, to determine whether the government's suspicions are "reasonable".

The government says that since it is not proving guilt or innocence, it is free to use anonymous, secret testimony which would not be permitted in a normal court, and to bar the suspects and their lawyers from attending hearings.

The suspects are theoretically free to leave Britain, but most are asylum seekers who say they will be tortured if they return home.

Most of the suspects have not been named, but they include the radical Islamic cleric Abu Qatada, the most high-profile figure held in a security sweep that followed the September 11, 2001, attacks in the United States.

Abu Qatada is wanted by Jordan for a series of bomb attacks in 1998, and British officials describe him as an "inspiration" for the September 11 attacks.

Britain has itself approached the United States over cases of British nationals interned without charges or trial at the U.S. navy base at Guantanamo Bay in Cuba. Recent U.S. plans for military trials of six foreigners -- including two Britons -- leading to possible death penalties provoked worldwide criticism.