MOSCOW (AFP) - Enigmatic in death as in life, Russian oligarch Boris Berezovsky has left a swirl of mystery after his passing, including a letter reportedly begging President Vladimir Putin for forgiveness.

The exiled businessman and former Kremlin powerbroker, who won asylum in Britain after falling out with Mr Putin and being convicted in absentia, was found dead at his Berkshire home at the weekend in circumstances that have yet to be made clear.

Russia's press on Monday leapt at the chance of publishing the wildest conspiracy theories, ranging from an assassination by MI5 to even a staged death that left the real Mr Berezovsky still alive.

Friends have suggested he was depressed over a costly court loss against fellow oligarch Roman Abramovich and personal trouble, pointing to suicide as a possible though unconfirmed explanation.

"Berezovsky made a mystery even of his death," said the mass circulation Komsomolskaya Pravda in allusion to his reputation from the 1990s as the shadowy grey cardinal of Kremlin politics.

"It seemed he liked blurring the boundary between truth and mystification," the business daily Vedomosti said, describing him as a "Faust or Frankenstein" who became a "King Lear".

A journalist for Forbes Russia magazine who spoke to Mr Berezovsky last week said the fallen oligarch had told the magazine that "life had lost meaning."

Adding to the intrigue was the apparent existence of a letter which Mr Putin's spokesman Dmitry Peskov said Mr Berezovsky had sent to the Russian president asking for forgiveness.

Mr Peskov told the Interfax news agency that the letter was a "personal one".

He also told TV Rain that the exiled oligarch had passed it on via a third party who he did not name.

The editor in chief of RT television, Margarita Simonyan, a key figure in Kremlin media circles, said on her television talk show on Sunday night that a figure close to Mr Putin had read out the letter to her.

She said that person, who she did not name, described the letter as a "prayer for returning to Russia".

"I committed many mistakes, I understand how hard it is to forgive me, but I had got confused and I beg for forgiveness," the person cited the letter as saying, Ms Simonyan said.

That person did not reveal the identity of the third person who gave the letter to Mr Putin but cryptically told Ms Simonyan: "You will never believe who handed it over."

Vedomosti said that a Kremlin official had unexpectedly revealed in a private conversation with one of its journalists this month that Mr Berezovsky had wanted to return to Russia and had written the letter to Mr Putin asking for an end to his prosecution at home.

Mr Vladimir Zhirinovsky, the head of the ultra-nationalist Liberal Democratic Party, said that he had met Mr Berezovsky by chance while on holiday in Israel earlier this year where the oligarch told him of the wish to return to Russia.

Mr Zhirinovsky, a veteran of the political scene for over two decades, told the pro-government Izvestia daily that Mr Berezovsky could have been killed by the British MI5 intelligence service after they found this out.

"It is possible that they accelerated his end. All the secret services have this tactic. Today there are many ways of provoking a heart attack," the maverick politician said.

Leading pro-Kremlin political analyst Vyacheslav Nikonov raised the possibility that Mr Berezovsky's "diabolical brain" could have even staged his own death.

"As long as there is no proof that he is dead, I am going to have doubts," he told First Channel television.

Adding further spice to the brew, the man Britain has accused over the polonium poisoning of the dissident Alexander Litvinenko, raised the possibility that foul play could have been a cause.

"Maybe I should not talk like this, but I knew that something was going to happen to him," former agent Andrei Lugovoi told RT, repeating his claim that Mr Berezovsky had been implicated in the Litvinenko murder.