The UK Government is trying to shut down a Website which it says is putting the lives of intelligence agents at risk by publishing their names.

Jon Silverman: "MI6 see this as a serious breach of security"

A government advisory body for the UK media has urged editors to seek advice before publishing details of the names and said officials were examining how the damage of the disclosure could be minimised.

Agents of the Secret Intelligence Service (SIS), formerly known as MI6, have been named in a list published on the US-based site.

Ex-MI6 agent chased

The government's Defence Advisory Notice Secretary, Rear Admiral David Pulvertaft, issued a notice to the UK media urging editors not to publish the address of the Website or details of its contents without first contacting him.

The BBC's John Silverman: "This is being taken very seriously indeed"

The government has been trying to shut down Websites set up by a former agent Richard Tomlinson since the end of last month. He was sacked by MI6 in 1995 and jailed in December 1997 for breaking the country's Official Secrets Act.

He planned to publish his memoirs in Australia and on the Internet. A seven-page synopsis included information on secret service training and methods.

Tomlinson was released on probation after six months of his one-year sentence and has been pursued around the world since by UK Government injunctions.

John Wadham: "Richard Tomlinson believes that the British Authorities are harrassing him"

His lawyer, John Wadham, told the BBC: "He's threatened to do the only thing he thinks he can do which is to reveal secrets and even those threats have no effect on the British authorities. He gave them fair warning and now he's taken this step."

Gave evidence at Diana inquiry

Tomlinson was arrested in Paris last July when he was preparing to publish more details about MI6. He was released again and went to New Zealand, where he was served with an injunction.

He later returned to Paris to give evidence at the inquiry into the death of the Princess of Wales, claiming that Diana's driver, Henri Paul, was working for British Intelligence.

He went on to live in Switzerland where he expressed support for former MI5 agent David Shayler, who also tried to release information about the security services over the Net.

Tomlinson 'persecuted'

In an interview with Channel 4 News, Mr Shayler said the UK Government should tread carefully with Tomlinson. "I think the government's running a great deal of risk because they have in some ways persecuted Richard Tomlinson."

The BBC's Chris Nuttall: "The Net tends to route round any kind of censorship"

Investigative journalist Duncan Campbell, who has been in touch with Tomlinson via e-mail, echoed this view.

He told the programme: "I think he's out to do damage because of the way he feels ... By making an international pariah of him they perhaps have driven him to the wall."

Swiss Website shut down

Tomlinson's Website maintained in Switzerland was shut down when the UK Government obtained an order against him on 30 April after he had threatened to publish sensitive information.

Last week, he set up another site with an American-based company, Geocities, intending to put up a map of MI6 offices worldwide and a directory of agents' names.

When Geocities was made aware of the Swiss injunction, it acted to prevent the publication.

The case reveals the difficulties governments face in preventing publication in the Internet age.