A flavour of the Cold War returned last night after Britain announced it would expel four Russian diplomats in response to Moscow's refusal to hand over the ex-KGB spy accused of murdering fellow former agent Alexander Litvinenko in London.

Vladimir Putin's government swiftly reacted by branding the move "immoral" and "provocative", warning there would be "the most serious consequences".

The expectation at Westminster is that Moscow will retaliate and announce tit-for-tat expulsions of UK staff from missions in Russia. Sources expected a response as early as this morning.

The prime suspect in the Litvinenko case, spy-turned-businessman Andrei Lugovoy, who has denied consistently any involvement in the murder, last night said: "I am really very sorry both Russian and British citizens are becoming hostages to such unprofessional and incompetent actions by British officials."

However, Mr Litvinenko's widow, Marina, welcomed Britain's move, saying she was "very grateful". She added: "It makes me proud to be a UK citizen, because I can see that my strong faith in the British authorities was well-founded and that they, too, share my determination. I believe Andrei Lugovoy should be brought to trial in the UK."

Prime Minister Gordon Brown, in Berlin last night, said Britain was right to expel the diplomats.

He added that he could not have "lawlessness" in London. "We believe there should be co-operation from the Russian authorities in this. We are sad that co-operation is not forthcoming. I have no apologies for the action we have taken."

Earlier in the Commons, Foreign Secretary David Miliband told MPs the government was "not seeking to be macho" but was responding robustly to a grave situation and sending the Russian government a "firm and clear" signal.

He said that, given the seriousness of the crime, Russia's refusal to extradite Mr Lugovoy for trial in England was "extremely disappointing". He said it was clear Russia had failed to appreciate the earnestness with which Britain took the Litvinenko case.

As well as the expulsion of four unnamed diplomats, Mr Miliband also mentioned a review of co-operation "on a range of issues" with an initial step of suspending talks on reforming visa arrangements between the two countries.

A departmental source said, under international convention, he could not disclose the seniority of the diplomats who were to be expelled. The expulsions will be the first since 1996.

Mr Litvinenko, a former Russian security agent who fled to Britain and became a British citizen, died in a London hospital last November from a fatal dose of the rare radioactive isotope polonium 210.

MPs were told that not only had Mr Litvinenko suffered a "horrifying and lingering death in front of his family" but hundreds of others had been put at risk of radiation as well.

The Metropolitan Police had assembled a "significant body of evidence" against Mr Lugovoy. In May, England's Crown Prosecution Service sought his extradition but on July 10 Russia's formal rejection arrived in Whitehall.