CHRONOLOGY: The case of poisoned ex-spy Alexander Litvinenko

(Reuters) - Britain said on Monday that it would expel four Russian diplomats in retaliation for Moscow's refusal to extradite Andrei Lugovoy, main suspect in the murder of Alexander Litvinenko, a former Russian intelligence officer living and working in Britain.

Russia said the decision was "immoral", would prompt retaliation and could only entail "the most serious consequences for Russian-British relations".

Here is a timeline of events:

November 1, 2006 - Litvinenko complains of feeling unwell after a day spent with contacts. After meeting the Italian Mario Scaramella at a sushi bar in London's Piccadilly, he saw former KGB contacts Lugovoy and Dmitry Kovtun at the Millennium Hotel in Grosvenor Square. Traces of radiation were later found at both places.

November 23 - Litvinenko dies of radiation poisoning. Traces of polonium-210 are found in his system.

November 24 - Litvinenko accuses Russian President Vladimir Putin of his murder in a statement read out by friends the morning after his death. Putin brushes off the accusation.

November 28 - British Prime Minister Tony Blair promises that "no diplomatic or political barrier" will be allowed to hamper the investigation into Litvinenko's death.

December 1 - Pathologists carry out a post mortem on Litvinenko's body.

-- Scaramella is admitted to hospital in London and traces of polonium-210 are also found in the urine of Litvinenko's widow Marina. Scaramella leaves hospital on December 6.

December 4 - British police fly to Moscow.

December 6 - British police say they are now treating Litvinenko's death as murder.

-- British police and investigators from Russian prosecutor-general's office question Kovtun in the same hospital where Lugovoy is being treated, apparently for radiation poisoning.

December 7 - Litvinenko is buried in London's Highgate Cemetery.

-- Russian prosecutors launch their own murder investigation. Prosecutors also open a criminal case into what they say was the attempted murder of Kovtun.

January 9, 2007 - Russia announces that Lugovoy was discharged from a Moscow hospital at the end of December. Lugovoy declined to say whether he had been contaminated with polonium-210.

February 1 - President Putin says that Litvinenko knew no official secrets and had no reason to leave Russia.