The Kremlin has threatened to retaliate after Dublin expelled a Russian diplomat in a row over the use of forged Irish passports by Russian agents in the United States.

The Government ordered the expulsion of the diplomat following a Garda investigation that concluded Russian intelligence agents stole the identities of six Irish citizens to produce fake Irish passports used by members of a espionage ring uncovered in the US last year.

Russian deputy foreign minister Vladimir Titov was quoted by Interfax news agency today as saying Moscow would retaliate. "This groundless and unfriendly act will not go without a corresponding reaction," he said.

In a statement yesterday, the Department of Foreign Affairs said the Garda inquiry had found that “there is an entirely persuasive picture of Russian intelligence service involvement in the manufacture and use of false documents based on the acquisition of details of six genuine passports belonging to Irish citizens”.

Garda sources told The Irish Times their report to Government concluded that a named Russian security agent based in Dublin had gathered, at the Russian embassy in Rathgar, details from Irish passports that were then effectively cloned in Russia for the US-based spies. Passports belonging to a married couple from Donegal and a female volunteer with the Irish charity To Russia With Love were among the six implicated.

The same Garda sources said a person working in the Russian embassy in Dublin was named in the Garda report as the suspect for stealing the passport information. The Department of Foreign Affairs then identified a named Russian diplomat for expulsion from Ireland.

Although gardaí are satisfied they have identified the right person, the suspect was never arrested and questioned because the situation was so “diplomatically sensitive”. An arrest was also regarded as imprudent because the suspect had diplomatic immunity.

Garda Commissioner Martin Callinan said his officers had established the facts of the case and had spelt “these out as clearly as we could” in the Garda’s report to Government.

The decision to expel the diplomat was taken at a Cabinet meeting yesterday. Russian ambassador Mikhail Timoshkin was later summoned to a meeting with David Cooney, the secretary general of the Department of Foreign Affairs. Mr Cooney informed him that the activities of Russian intelligence agents as outlined in the Garda report were “completely unacceptable and not the behaviour the Government would expect from a country with which we have friendly relations”.

The diplomat concerned is expected to leave Ireland within weeks. A spokesman for the Russian embassy said it would be making no comment on the matter.

The Department of Foreign Affairs said Ireland regretted the need to expel a diplomat. “The Government, by today’s action, has once again made clear that it will not tolerate the fabrication and use of forged Irish passports by agents of a foreign State,” it said.

US investigators broke up the spy ring last June with the arrest of 10 people in New York, Boston, New Jersey and Virginia. All 10 were later deported as part of a spy swap deal with Russia.