Britain's ambassador has issued a direct challenge to Russia over its refusal to extradite the man accused of killing Kremlin critic Alexander Litvinenko in London.

Diplomatic relations with Russia will come under increasing strain today amid promises of retaliation for the expulsion of four of its UK diplomats.

Russian authorities have formally refused to extradite Andrei Lugovoy to face trial over the murder of Alexander Litvinenko.

Russia has refused Britain’s request to extradite a suspect in the Alexander Litvinenko poisoning case, news reports said today.

Russia’s security agency today said that it has launched an investigation into suspected British spying, based on information from Britain’s chief suspect in the fatal poisoning of former agent Alexander Litvinenko.

Russia’s security agency has opened an espionage case in connection with claims made by Andrei Lugovoy, the suspect named by Britain in the radiation poisoning death of former agent Alexander Litvinenko in London last year.

The widow of poisoned former KGB agent Alexander Litvinenko says he was not working for British intelligence, as his alleged killer claims.

The chief suspect in the killing of ex-KGB agent Alexander Litvinenko today claimed that British intelligence services had a hand in the poisoning – an assertion likely to further damage relations between Moscow and London.

Russia's top prosecutor told his British counterpart today that Russia could prosecute the chief suspect in the Alexander Litvinenko killing if Britain presents sufficient evidence, the Interfax news agency reported.

Russian businessman Andrei Lugovoy today said that British prosecutors' decision to charge him with the murder of Alexander Litvinenko is "politically motivated", news agencies have reported.

The Russian prosecutor-general’s office today said it would not turn over Andrei Lugovoy, charged by Britain in the poisoning death of Alexander Litvinenko, the Interfax news agency reported.

An ex-KGB agent is to be charged in connection with the murder of poisoned former Russian spy Alexander Litvinenko, the Crown Prosecution Service said today.

Prosecutors in the UK investigating the case of poisoned Russian spy Alexander Litvinenko will make an announcement today, the Crown Prosecution Service (CPS) said.

Two exiled Kremlin adversaries refused to answer key questions posed by Russian authorities probing the poisoning death of former security agent Alexander Litvinenko, a Russia investigator said in an interview published today.

Russia complained today about a “long delay” in the British response to a request to send investigators to London for inquiries into the poisoning of former spy Alexander Litvinenko.

The head of a private centre in Russia that trains the country’s special forces today admitted that photographs of exiled former spy Alexander Litvinenko had been used during target shooting practice.

The man reported to be a suspect in the murder of a former Russian agent in London hit out today at “lies, provocation and government propaganda”, denying any role in the radiation poisoning death of Alexander Litvinenko.

Russia's top prosecutor said his office wants to question fugitive tycoon Boris Berezovsky the probe of ex-security agent Alexander Litvinenko's poisoning death, Russian news agencies reported today.

Russia’s top prosecutor said British investigators have asked to return to Moscow as part of their probe of ex-security agent Alexander Litvinenko’s poisoning death, Russian news agencies reported today.

Italian prosecutors were today due to question an Italian man who met with a former Russian spy in London the day the ex-KGB agent was poisoned.

Police today arrested an Italian security expert who met former Russian spy Alexander Litvinenko the day he fell ill from poisoning.

Police have arrested Italian Mario Scaramella, who met former Russian spy Alexander Litvinenko the day he fell ill.

A key witness in the poisoning death of former Russian security agent Alexander Litvinenko today faced a second round of questioning by British investigators, news reports said.

An ex-business associate of Alexander Litvinenko said today that the former Russian spy was murdered because of information he held on a powerful Kremlin figure.

A key witness in the investigation into the radiation death of former Russian intelligence agent Alexander Litvinenko denied involvement in the poisoning today, saying he is being unjustly accused in the media.

Interpol has joined the investigation into the killing of former Russian agent Alexander Litvinenko, the Interfax news agency reported today.

Six Irish citizens are being sought to undergo tests for possible radiation poisoning amid the on-going investigation into the death of a former Russian spy in London.

Six Irish people who stayed at a top London hotel the same time Russian spy Alexander Litvinenko was poisoned are to be tested by radiology experts, it emerged tonight.

The former wife of a Russian businessman in the focus of investigations surrounding the fatal poisoning of ex-spy Alexander Litvinenko, along with three relatives, are being tested for radiation contamination, German police said today.

German airline Germanwings said today that an aircraft on board which a contact of dead Russian spy Alexander Litvinenko flew to London was being tested for possible traces of the radioactive substance polonium-210.

German police said today that they had found traces of radiation at two sites in and near Hamburg linked to a contact of poisoned former Russian spy Alexander Litvinenko.

Russia plans to send investigators to Britain in connection with the probe into former security agent Alexander Litvinenko’s death, the chief prosecutor’s office said today.

Russian businessman Dmitry Kovtun, a key figure in the poisoning investigation of ex-security agent Alexander Litvinenko, is in a coma, a Russian news agency said today, but a lawyer close to the case denied the report.

The funeral of the poisoned Russian former spy Alexander Litvinenko was being held at Highgate Cemetery in north London today.

Russia has opened a criminal case in the poisoning death of ex-spy Alexander Litvinenko and for the attempted killing of associate Dmitry Kovtun, the Prosecutor General’s office said today.

Ex-Russian spy Alexander Litvinenko, who died last month after being poisoned, was scheduled to be buried in London today, his friends said.

Russian investigators said they have begun questioning in the radiation poisoning death of former Russian agent Alexander Litvinenko, a case that has left radiation traces from a London soccer stadium to the British Embassy in Moscow.

Experts have found minor traces of radiation at the British Embassy in Moscow but they presented no health risk to the public, an embassy spokesman said today.

The Italian academic who met Alexander Litvinenko on the day he was allegedly poisoned has been discharged from hospital, it was disclosed today.

Russia will not extradite possible suspects in the poisoning of ex-spy Alexander Litvinenko to Britain, the prosecutor general said today.

A team of experts from Britain investigating the death of former spy Alexander Litvinenko are to carry out “precautionary tests” for radiation at the British Embassy in Moscow, the British foreign office said tonight.

Russia today warned that relations with Britain could be damaged if speculation continued to suggest that they were officially involved in the poison spy case.

British coroners investigating the poisoning death of a former KGB spy have officially requested assistance from their Russian counterparts and are soon due in Moscow to collect information, prosecutors said today.

British Home Secretary John Reid today said he was to reassure European Union counterparts that there were “minimal” health risks for other nations in wake of the poisoning and death of a Russian ex-spy in London.