Many years have passed but the picture is still clear in my mind. I am sitting at the kitchen table with my mother. She's having chicken soup but I'm not. I'm fasting. Not to eat and drink for a whole day is no easy matter for a 15-year-old, and it's just past two in the afternoon. It's Yom Kippur, 1973, Israel. Then someone is calling my mum's name from under our balcony. I recognise the voice. It's Mazal, our next-door neighbour. Mum takes her plate with her and steps out on to the balcony to hear her. But it's not necessary; Mazal is quite excited. "Turn the radio on," she screams at the top of her voice. "Turn the radio on! War has just broken out."

So traumatic! So out of the blue! So unlike 1967, when our troops defeated Arab armies quickly and elegantly. Back then, my dad woke me up one morning, pointing at a picture in a half-folded newspaper, telling me - a sleepy nine-year-old - that the man in the picture would soon lead us to a great victory over the Arabs. I remember looking at the picture and seeing someone with a black eye-patch who looked like a pirate; as a young boy, I knew that pirates always win wars, and I was happy enough.

But now, six years on, the "pirate" Moshe Dayan, is on our black-and-white TV screen, defeated, his head bowed and his voice trembling as he tells us that our troops at the front are fighting an invading enemy and that "we are fighting for our lives". Fighting for our lives? Us?

In subsequent years, the Yom Kippur trauma continued to deeply affect our collective life. Losing close to 3,000 young men touched upon every corner of the then small Israeli community. And, as I grew up and moved from high school to army to university, and then turned my attention to the history of Israel's wars and even penned a few books on the subject, I realised that, of all Israel's wars, the Yom Kippur war was the most exciting. A superb, Arab surprise attack succeeds in catching Israel unprepared, and a near-miraculous Israeli comeback and counter-attack brings Israeli troops to the gates of Cairo and Damascus.

Above all, it is a John le Carré-style espionage drama, at the heart of which is a colourful super-spy, whose name is shrouded in mystery and is top secret in Israel. Little did I know at the time that I would be the person to unmask the mysterious spy and it would all end in tears.

Back in 1969, a man with an Egyptian accent telephoned the Israeli embassy in London, saying he wanted to work for Israeli intelligence. Although he identified himself, his name didn't ring a bell, and it ended there. But he tried phoning again and, this time, the penny dropped. The shocked Israelis met him the next day at a Park Lane hotel, and recruited him for the Mossad.

The information this Egyptian spy began to provide the Israelis with was extraordinary. It included original maps and war-plans of the Egyptian army and detailed descriptions of how it would attempt crossing the Suez Canal in a future war.

The entire Israeli thinking before the Yom Kippur War was built on the basis of the information provided by this Egyptian spy. It came to be known as "The Concept" and at its heart was the view that Egypt would not attack Israel without first obtaining from the Soviets Scud missiles and bombers, which could enable the Egyptians to threaten Tel Aviv. All Israel had to do was keep an eye on transfers of such weapons from Moscow. The Israelis dubbed their spy "The Angel".

Like other scholars of Israel's wars, I knew that, once upon a time, there was a "miraculous" Egyptian spy who had worked for Mossad before the Yom Kippur war. In my early works on Israel's wars, I had hardly mentioned him as I had so little to add. But, gradually, I became fascinated with this spy, particularly as I had heard rumours that some in the Israeli establishment believed that this "Angel" was a double-agent who duped Israel. They said that he led Israel to believe in "The Concept" - that Egypt will not go to war without certain weapons at its disposal - but he later failed to let Israel know that Egypt decided to attack anyway, without getting the special weapons, thereby catching Israel off-guard on Yom Kippur.

Intrigued by the idea of a double-agent at the heart of the Israeli military establishment, I embarked on a campaign to uncover the truth behind the mysterious spy. I collected every scrap of information - not that there was much - read all that was to be read on the Yom Kippur War, and made lists of potential spies.

As I pored over the documents and memoirs, I reduced my could-be spy names to a shortlist. Bit by bit, I managed to delete names until I was left with a single name: Ashraf Marwan, the son-in-law of Egypt's President Gamal Abdel Nasser and, after the death of Nasser in 1970, the right-hand man of his successor, Anwar Sadat.

I then started to drop hints about the identity of Marwan, leaving myself just enough room to be able to withdraw should Marwan deny it, or - more importantly, as he was a rich man - if he threatened libel action. In a chapter on the Yom Kippur War in my 2002 book, A History of Israel, without spelling out his name, I wrote that "he was a very close family member of Egypt's President Nasser" and that in Israel he was known as "The Son-in-Law".

By now, I was convinced that he was a double-agent who misled his Israeli employers and hinted as much. From one of my students, an Egyptian who knew the Marwans, I obtained Ashraf Marwan's London address. I sent him a copy of my book, and marked the pages where I hinted that he was a Mossad spy and a double-agent, and dedicated that copy of the book: "To Ashraf Marwan, Hero of Egypt".

It wasn't difficult to surmise that the spy was Ashraf Marwan, Nasser's son-in-law. Indeed, on 2 December 2002, the Egyptian newspaper Sawt al Ummah had asked Marwan if he was in fact the person at the heart of my story. Marwan replied indirectly by saying that my book was a "stupid detective story". My response was to give a counter-interview to an Egyptian newspaper in which I unmasked Marwan as the mysterious spy, spelling out his name and adding that I was convinced that he was a double-agent who successfully managed to trick Israel by deceiving it over the "Concept".

Soon after my interview was published in Egypt, my phone rang and the person on the line identified himself as "the man you've written about". When I asked him to prove that it was indeed him - Ashraf Marwan - he simply said: "You've sent me the book with the dedication." This was the beginning of a bizarre relationship, as Marwan and I kept in touch for almost five years. We met, were constantly on the phone, and frequently discussed the Yom Kippur War, as Marwan made me a consultant on his memoirs. But then came a most dramatic event.

On 26 June 2007, a shaken Marwan left three telephone messages on my answering machine asking me to phone him urgently. I phoned him in the afternoon and we scheduled a meeting for the next day near King's College in London, where I teach.

The meeting never took place. Around the time we were due to meet, Marwan's body was found in the private rose garden below his flat in central London. Did he jump, or was he pushed? A three-year police investigation failed to offer an adequate answer.

Exposing the identity of the greatest spy Israel ever had was a big mistake, particularly in the light of Marwan's tragic death. But my revelations opened the gates to a wave of books and articles on the Yom Kippur War and unleashed a fierce debate, particularly in Israel, about the role Ashraf Marwan played in the period leading up to the Yom Kippur war. Was he Israel's best spy ever, as his Mossad employers still insisted? Or was he - as I believe - the jewel in the crown of the Egyptian deception plan before that war? 

Ahron Bregman's books include 'The Spy Who Fell to Earth:

My Relationship with the Secret Agent who Rocked the

Middle East'