He was accused of being both an Israeli spy and an Egyptian double agent. Three years after his death, Ashraf Marwan's career and mysterious death in London remain among the most intriguing unsolved riddles of modern espionage.

The billionaire arms dealer, who was the son-in-law of Egypt's second president, fell to his death from a fifth-floor West End balcony on a summer's day in 2007. His death in the heart of wealthy London made world headlines.

An inquest this week will attempt finally to unravel the circumstances of Marwan's fatal fall. In an exclusive interview, his widow has told the Observer that in the days before he died her husband believed his life was in danger. After Marwan died, his family discovered that the draft manuscript of his memoirs – which threatened to expose secrets of the Middle East's intelligence agencies – had disappeared from his bookshelf.

Mona Nasser, Marwan's wife of 40 years and one of two daughters of the former Egyptian president Gamal Abdel Nasser, said that her husband confided that he was being pursued by assassins nine days before his death. She believes he was killed by the Mossad, Israel's intelligence agency, and is expected to be a witness at the coroner's inquest that opens on Monday.

Nasser has also criticised the investigation by the Metropolitan Police into Marwan's death as negligent. The shoes that Marwan was wearing when he died – which may have provided vital DNA evidence to show whether he was murdered or jumped – were lost by investigating officers.

Since his death, there has been intense speculation over the secretive life of Marwan and his role in the Yom Kippur war, waged between Israel and a coalition of Arab states backing Egypt and Syria in 1973. Mossad agents say Marwan was their heroic spy at the heart of the Egyptian government. But both Hosni Mubarak, Egypt's current president, and the former head of Israeli military intelligence have indicated that Marwan was a double agent feeding misinformation to the Israelis.

Marwan, 63, was found dead in June 2007 on the pavement beneath his exclusive Carlton House Terrace flat, a stone's throw from Trafalgar Square. At least one witness claims to have seen two men of Middle Eastern appearance on his fifth-floor balcony seconds after he fell.

Nasser, speaking from her home in Cairo, said that Marwan told her three times in the four years before he died that his life was in peril. The last time he did so, they were alone together in their London flat. "He turned to me and said: 'My life is in danger. I might be killed. I have a lot of different enemies.' He knew they were coming after him. He was killed by Mossad," she said.

Those fears were at their most intense just days after an Israeli court ruled that Major-General Eli Zeira, who headed Israeli military intelligence during the 1973 war, had exposed Marwan's identity as an Israeli spy. "I was worried, of course, but in our life together we had been in many dangerous situations. He was determined to carry on as normal," she said.

Nasser met Marwan, a tall, handsome chemist, in August 1965 in Cairo, where they were both students. He was, she said, charming and quietly spoken. They were married the following year. He soon began working closely with her father. "My father would send my husband to missions in foreign countries. My husband refused to give me sensitive information about his trips because that would have put me in danger," she said.

Speaking of her husband's alleged role in the Yom Kippur war, Nasser said that she confronted her husband in 2003 but he had denied that he had any direct contact with Mossad agents.

However, since Marwan's death Nasser said she had learned from Egyptian intelligence officers that he did have a role in feeding Mossad with false information. She still does not feel as if he lied to her. "My husband was a hero who served his country. He only did what was asked of him to perfection," she said.

After Anwar Sadat, who had succeeded Nasser as president in 1970, was assassinated in 1981, Marwan and his family moved to London.

Nasser said that the police investigation following her husband's death had been a sham. "The investigation was extremely negligent. They did not seal the area properly. They did not take fingerprints. They lost the shoes he was wearing when died. This was all vital information," she said.

The shoes may have carried clues as to how he died, because he suffered from neuropathy in his feet. This meant that he could not lift them higher than a few inches without help. "If he was supposed to have climbed over a metre-high balcony rail, there would have been scuff marks," she said.

A police spokesman said that the three-year investigation into Marwan's death, which was removed from one set of detectives and handed to the Specialist Crime Directorate after Marwan's shoes were lost, continues.

The coroner's inquest has been scheduled to last for at least three days, and is expected to hear testimony from police officers and from former business partners of Marwan.

On the day that Marwan died, he had been working hard on his memoirs of his role in the 1970s, according to family members. His wife said the lock on the front door had been left on the latch by one of their household staff. A housekeeper in the flat was the only other person present in the 15-room apartment, giving an intruder ample time to find Marwan and kill him, she claimed.

"I believe that the intruders took him to the bedroom, they hit him and they threw him out of the window over the balcony. Someone on a fourth-floor balcony who gave evidence to the police heard him scream before he fell. Do people committing suicide scream before they fall?" she said.

Nasser said that the couple had been looking forward to holidays with their five grandchildren, and had made many plans for the long and short term.

"He was happy. We were happy. There is no way he killed himself. It is so painful to think about one's husband being thrown over a balcony. It is so horrible. I am talking about it for the first time because the truth should come out," she said.