Scotland Yard detectives are said to be preparing to issue arrest warrants against three former KGB officers suspected of poisoning Russian spy Alexander Litvinenko, according to a report in a British paper.

The move could further strain the relationship between Downing Street and the Kremlin, which is likely to block any extradition requests, The Mail On Sunday reports.

Police have apparently told Mr Litvinenko's widow Marina they plan to lay murder charges against the suspects — Andrei Lugovoi, Dmitri Kovtun and Vyacheslav Sokolenko.

All three former agents, who met the victim at a London hotel three weeks before his death, deny any involvement in the poisoning plot. Traces of radiation were later found at the hotel and at other sites across the capital, as well as in Germany and Russia.

Mr Litvinenko, who was a vehement critic of Russian president Vladimir Putin, died in a London hospital in November after being poisoned with radioactive Polonium 210.

His wife and friends branded his death as "state-sponsored terrorism" carried out on the orders of the Russian president. The Kremlin said the claims were ridiculous.

According to sources, detectives now have enough evidence to bring charges "within three weeks".

In December, nine Scotland Yard detectives flew to Moscow to conduct their investigation. They were not allowed to interview the suspects, but were present when Russian police quizzed them.

Britain has no extradition treaty with Russia, meaning any trial will probably have to be held in Moscow.

Click here to read the full Sky News report.