For 12 years, he took part in secret operations in Europe and the Middle East as an agent for Israel's intelligence agency, the Mossad. He played a key role in a high-profile assassination fiasco in the 1990s. He was required to manipulate and deceive. He experienced the acute loneliness of an undercover operative in situations where only he knew his true identity.

Then Mishka Ben-David quit as a spy and became a bestselling, prizewinning thriller writer. Now he has made his literary debut in the UK following the translation of one of his five spy novels into English. Ben-David will travel to London next week to promote the book and talk about his experiences.

Duet in Beirut centres on an agent who is blamed for the failure of an operation to eliminate a Hezbollah leader after he balks at pulling the trigger when the target's daughter unexpectedly appears. Thrown out of the Mossad, the agent decides to finish the job alone, but his former commander is determined to stop him. "Things that go wrong, or moral dilemmas, or huge conflicts – this is the material of novels," Ben-David told the Observer at his home just outside Jerusalem.

"In fact, the ratio of failure to success is about one to 1,000, according to my calculation." Nevertheless, the former spy's career included close-up experience of a catastrophic operation.

In 1997, Binyamin Netanyahu, then in his first term as Israel's prime minister, ordered the Mossad to eliminate Hamas's leader, Khaled Mashaal. The Mossad unit chose to poison Mashaal in the Jordanian capital, Amman.

The deadly substance was administered to Mashaal's left ear by two Mossad agents, but the men were unexpectedly arrested. As the Hamas leader lay close to death in hospital, Jordan's King Hussein furiously demanded that Israel save Mashaal's life or face serious consequences, including the execution of its agents.

Ben-David was in an Amman hotel, holding the vial containing an antidote to the poison. "I was told to take the antidote to the lobby of the hotel and hand it over to a representative of the Jordanian security services. I went down and met a Captain Firas. We exchanged hardly any words. I just gave it to him." Ben-David thought it was the right decision to hand over the antidote, rather than sacrifice two operatives. "Every failure has an impact. It was damaging to the Mossad's prestige. For several months there were inquiries, and it's difficult to operate under such circumstances." He defends assassinations as a way of reducing the number or scale of attacks. But he adds: "Assassinations did not and cannot solve the conflict; that can only be done by negotiations and concessions on both sides."

Ben-David was recruited to the Mossad after answering an opaque advertisement placed in an Israeli newspaper. At the time he was in the midst of a PhD in Hebrew literature. "But I was looking for something more intriguing and meaningful." Following a meeting at which his interlocutors did not disclose their identities as Mossad agents, he spent a year undergoing interviews and psychological and practical tests. "All the time you are being challenged, stretched to the very end of your capacity."

After 12 years as an agent, he decided to reconnect with normal life, spend more time with his family and return to writing. There is, he said, "nothing I regret" about his Mossad service. "Ninety-nine per cent of what I did were things that would not raise any moral dilemmas. The few times that I raised issues, we found solutions. Nothing troubles my conscience."

Duet in Beirut, by Mishka Ben-David, is published by Halban Publishers