An Egyptian billionaire financier who feared for his life after being accused of being a Mossad spy was found dead outside his Mayfair flat yesterday in suspicious circumstances.

Ashraf Marwan, the son-in-law of the late President Gamel Abdel Nasser, was found beneath his fourth-floor flat in Carlton House Terrace.

Police were treating his death as suspicious. Friends of Mr Marwan, a former shareholder in Chelsea Football Club, said that he had feared assassination after being named three years ago as an agent during the Yom Kippur war.

Rumours of his death circulated in London’s Arab community last night. Some believe that he may have taken his life after a serious illness was diagnosed.

Mr Marwan’s death will send shockwaves across the Middle East and among some of Britain’s wealthiest people. His associates included Adnan Khashoggi, the arms dealer, Ken Bates, the football club chairman, the Libyan leader Colonel Muammar Gaddafi and the late Tiny Rowland.

If found to be murder, his death will carry echoes of last year’s assassination of Alexander Litvinenko, the former KGB agent.

Mr Marwan, 63, was identified as an agent by the Vanity Fairwriter Harold Bloom in his book Eve of Destruction, which detailed his involvement in the Yom Kippur war in 1973. Although Nasser, who humiliated the British Government at Suez, died in 1970, Mr Marwan, his son-in-law, was part of the inner circle of his successor, Anwar Sadat, who started the war.

The identity of the agent, described by a postwar Israeli inquiry commission only as “the source”, had been a closely guarded secret. Evidence pointed towards someone high in the Egyptian Establishment.

From published accounts based on Israeli sources, it was alleged that Mr Marwan was a “walk-in” who entered an Israeli embassy in Europe and offered his services in 1969. Extensive checks convinced Mossad that he was not a double agent.

In the ensuing years Mr Marwan provided information on Egypt and the Arab world that Moshe Dayan, the Israeli Defence Minister, and others would later term priceless.

Some believed that he was a double agent. Mr Marwan denied the claims, saying that he had never worked for the intelligence communities on any side. Mr Bloom acknowledged that he could be assassinated.

Mr Marwan was believed to have been born in Egypt into a wealthy family. He married Nasser’s daughter Muna in the 1960s and they had two sons and a number of grandchildren.

Rumours of how he made his money have circulated within the Arab community for many years. Many say that he was an arms dealer and had been introduced to plenty of contacts by Sadat.

Mr Marwan considered London his main home, according to friends, despite owning property across the world. Standing at 6ft 2in and very thin, he was seen at a social gathering in Central London last week with his wife.

Mr Marwan was a friend of Ken Bates, the former Chelsea chairman, and at one point held a 3 per cent share in Chelsea Village, one of the holding companies of the club. Later he was believed to be the subject of a Financial Services Authority inquiry into the sale of shares to Roman Abramovich, the current owner. He was also believed to have owned the Son Vida, one of the best hotels in Majorca.

Last night police kept a tight cordon around Carlton House Terrace, a white Grade I listed building overlooking St James’s Park that stretches between Buckingham Palace and Horse Guards Parade. Friends of the family said that Mr Marwan’s wife was flying back to Britain from Egypt.

A Scotland Yard spokesman said last night: “We were called at around 13.40 to Carlton House Terrace. The death is being treated as suspicious at this stage and the inquiries are under way.”

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