A businessman who met Alexander Litvinenko on the day he was poisoned is said to be critically ill this evening, just hours after being questioned by police in Moscow, in a further twist to the bizarre saga.

Dmitry Kovtun was with his friend Andrei Lugovoy when they met Litvinenko at the Millennium Hotel in Grosvenor Square, London on November 1.

Police in London have been following the trail left by both men across the capital, which include a number of locations where the radioactive isotope polonium-210 has been found.

Mr Kovtun was interviewed by Russian prosecutors yesterday in the presence of the British detectives.

The Russians announced today that they had opened their own criminal investigation into the Litvinenko poisoning and also believe that both Mr Kovtun and Mr Lugovoy were both poisoned.

Tonight the Interfax news agency, quoting unnamed sources, said that Mr Kovtun was in a critical condition in a Moscow clinic. "Doctors have classified Kovtun’s condition as critical," the source said. There has been no independent confirmation of the report.

The news came on the day that Litvinenko was buried in the same London graveyard as Karl Marx today after the Muslim prayer service that he asked for as he lay on his deathbed last month.

Around 50 of Litvinenko’s friends and family gathered for his burial at Highgate Cemetery in North London. Marx, the German father of Communism who also died in exile, is buried close by.

The mourners were led by Litvinenko's wife, Marina, and 12-year-old son Anatoly. Joining them in the rain was his father Walter, his mother Nina Belyavskaya and first wife Natalia, as well as friends Alex Goldfarb, the exiled Russian billionaire Boris Berezovsky and the filmmaker Andrei Nekrasov.

Litvinenko’s dark-stained coffin was driven around the perimeter of the cemetery before being laid to rest at the burial site in a non-denominational service. The Health Protection Agency has been consulted on the necessary safety precautions for his coffin because his body remains radioactive.

Earlier, Walter Litvinenko joined mourners at the Central London mosque in Regent’s Park for a Muslim service dedicated to the former spy. His father says he converted to Islam shortly before his death, although some of his friends say he was simply sympathetic to the cause of Muslim Chechens.

But critics of President Vladimir Putin fear that the announcement that the Russian Prosecutor General’s office is to conduct its own investigation into both the Litvnenko and Kovtun cases could severely hamper the British one.

The move would allow suspects to be prosecuted in Russia. Officials previously have said that Russia would not extradite any suspects in the killing of Litvinenko, who died in London on November 23.

Several workers at the Millennium Hotel in London's Grosvenor Square where Litvinenko held a meeting also tested positive for low levels of polonium-210. Health officials are likely to ask anyone who was in the hotel's Pine Bar on 1 November to come forward.

In a statement written on his deathbed, Litvinenko, a 43-year-old former KGB officer, accused Mr Putin of being behind his poisoning, an accusation echoed once more by his father today.

On Monday, Sergei Lavrov, the Russian Foreign Minister, expressed worries that ties could be damaged if insinuations of high-level Russian involvement continued.

But today he insisted that political ties between Britain and Russia were not being affected by the investigation into the poisoning. He said: "The Scotland Yard investigation is not affecting the quality of Russia-Britain political relations."

A lawyer representing Mr Kovtun and Mr Kovtun, Andrei Romashov, emphasised that they were being treated "as witnesses". He disclosed that Mr Kovtun had also been interviewed briefly on Tuesday about his two trips to London.

A third Russian businessman, Vyacheslav Sokolenko, who also flew to London to watch a football match on November 1, is on the list of men British detectives want to meet.

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