UK is attacked in Litvinenko row

Andrei Lugovoi has denied involvement in the murder

A senior Russian official has attacked the UK's decision to expel four Russian diplomats as "groundless, inappropriate, unjustified".

The Russian prosecutor general's office said the UK's decision over the expulsions was politically motivated.

The UK expelled the four after Russia refused to extradite Andrei Lugovoi - accused of killing ex-KGB agent Alexander Litvinenko in London.

Prosecutor Alexander Zvyagintsev said the refusal was "on the basis of law".

KEY EVENTS IN CASE 1 November 2006: Alexander Litvinenko meets Andrei Lugovoi and another Russian at a London hotel 23 November 2006: Litvinenko dies in a London hospital 24 November 2006: A Litvinenko statement accuses Russian President Vladimir Putin of involvement in his death. Experts say Litvinenko was poisoned 6 December 2006: UK police say they are treating the death as murder 22 May 2007: Lugovoi should be charged with Litvinenko's murder, British prosecutors say 28 May 2007: UK makes formal request for Lugovoi's extradition from Russia Full timeline of events

Under the European Convention on Extradition 1957, Russia has the right to refuse the extradition of a citizen, and its constitution expressly forbids it.

Mr Zvyagintsev also said in a press conference that the Russians had "grounds to doubt the objectivity of the British justice system".

"There is no evidence in the materials provided by Britain that there was an objective investigation of the Litvinenko case by Scotland Yard," he added.

Russian prosecutors would open a case against Lugovoi if there was evidence to support such an investigation, he said.

Mr Zvyagintsev said he had not ruled out the involvement of "persons from Litvinenko's closest London circle".

Last week, four Russian embassy staff were expelled from the UK after the British extradition request was refused.

That prompted Moscow to retaliate by asking British embassy staff to leave within 10 days.

Mr Litvinenko, who had taken UK citizenship, died of exposure to radioactive polonium-210 in London in November 2006.

Traces of the radioactive isotope were found in several places visited by Mr Lugovoi, who denies any involvement in Mr Litvinenko's death.