All I ever wanted to do was to work as a journalist and set up my own newspaper. Instead, I became a spy for the security services in London, reporting on the radical Islamic clerics who were indoctrinating recruits to train as terrorists. To my mind there was little difference between being a journalist and an agent. As things turned out, it was not as simple as that.

My induction into this secret world took place in 1997 at a French restaurant in Knightsbridge, which was a convenient location for my host, a senior diplomat who worked nearby at the French embassy. Known to me by the codename “Jerome”, he was head of the London section of the Direction Générale de la Sécurité Extérieure (DGSE), the French equivalent of Britain’s MI6.

I had reached a crossroads in my life and felt lost. I was a 36-year-old Algerian journalist whose attempts to create newspapers in Algiers and France had been frustrated by the violent factionalism that was tearing my country apart. Seventy of my colleagues and friends had been murdered, and I had arrived in London as a refugee in 1994 with my wife and young son.

My English was poor and my status uncertain, as I had neither a British nor a French passport and my application for asylum had been refused. My only assets were my investigative skills and my fluency in French. I felt driven to help stop the extremists who were killing hundreds of Algerians in daily bloodbaths. So I approached the DGSE at the French embassy.

As I listened to Jerome over flame-grilled steak and a bottle of red wine, it was clear he was preoccupied with the terrorist threat to the 1998 World Cup in France.

The threat came from extremists in London, but the DGSE was powerless to act. Worse, the British security service did not listen to French warnings. Only later, when I was working for Special Branch and MI5, did I realise the extent of those services’ wilful blindness.

The French wanted me to infiltrate the Islamic fundamentalist community in Britain to gather evidence of any plot.

The prime target was a radical Muslim cleric based at a London mosque. The mosque had been taken over by Algerian fundamentalists who had installed this cleric as the spiritual leader of the GIA, the terrorist group dedicated to installing an Islamic republic in Algeria and responsible for the carnage in my country.

I posed as a GIA supporter, managing to convince those who had known me as a journalist in Algiers that I had turned against the government. I had been in the habit of attending the mosque with friends to pray, so I had a good idea of who the extremists were.

The cleric was a fiery orator who made little secret of his indoctrination. He would sit with young British Muslims who had already been recruited by staff. It was like a finishing school for those who had agreed to go on jihad. The recruits, who were from all backgrounds and classes, were subjected to his very powerful rhetoric.

The message was all about jihad, killing and going to paradise. They would be asked: “How long are you going to live — 60, 70, 80 years? Then you die and go to hell. But you should think about eternal life. To go to paradise you have to use a sword, meaning you have to kill in the name of Allah.”

This brainwashing was so incessant that had I not been so strongly opposed to them, I would have become a terrorist myself. After spending hours in the mosque every day, I went to the pub just to erase what had been put into my mind. I was not too worried about my safety. My main concern was for my wife, from whom I was divorced, and our son, but I knew they were being looked after.

I was not only reporting back to my DGSE contact over our lunches in Piccadilly and Knightsbridge but handing over documents that I had stolen from the mosque’s offices. In 1998 the French approved my plan to build a network that would infiltrate hardcore groups in London.