LONDON (Reuters) - The head of the U.S. Federal Bureau of Investigation waded into a sensitive British security debate on Monday by arguing strongly in favor of the use of phone-tap evidence to prosecute terrorist suspects.

FBI Director Robert Mueller prepares to testify at a House Appropriations Committee hearing on the FBI's 2009 budget on Capitol Hill in Washington April 1 2008. REUTERS/Jason Reed

Almost alone among Western countries, Britain does not allow suspects’ telephone conversations, intercepted by the police or the MI5 intelligence agency, to be presented as evidence to a trial jury.

The government last year announced a review of the policy but has yet to say whether it will change it.

“I know that is a debate here,” FBI Director Robert Mueller told London’s Chatham House think-tank, describing the use of intercepts in court as “tremendously beneficial”.

“Some of the best evidence you get are the words coming out of the mouths of those who are being prosecuted. I understand some of the ups and downs on that but you really put your intelligence and law enforcement services in a bind” by ruling such evidence inadmissible, he said.

For the authorities, Mueller said, this meant that “you have to continue with the ongoing threat until such time as you get admissible information”.

They then had to “hope and pray” that police searches after the arrest of suspects yielded enough evidence to complete a prosecution case, which Mueller called “an untenable situation in my mind”.

The British government announced a review of the current law last June, but Prime Minister Gordon Brown told parliament on February 1 it had found that “further extensive work” was needed to see how intercept material could be used without jeopardizing sensitive techniques or draining resources.

British law does allow the use in court of material gathered by bugging suspects’ homes or vehicles, but not from tapping their phones.

Mueller’s comments, to an audience including the head of MI5, came in response to a questioner who asked how the United States was able to combat terrorism with a legal system that allows law enforcement officers just two days to question suspects before charging them.

Britain allows police to hold suspects up to 28 days before charges are made. The government wants to increase this to 42 in extreme situations, for example involving complex plots with extensive overseas links.

Apart from the intercept issue, Mueller cited the use of plea bargaining to persuade suspects to divulge intelligence on cells and plots.

“There is not a terrorist group that we have taken down since September 11 (2001) in which one or more of them have not decided to cooperate. And from that cooperation comes a vast pool of intelligence,” he said.