The Shin Bet wasted a golden opportunity when it turned against one of its most high-profile Palestinian agents, a former member of Israel's domestic intelligence agency has argued.

Gonen Ben Yitzak was the handler of Mosab Hassan Yousef, the son of West Bank Hamas chief Sheikh Hassan Yousef, who was recruited to work for Israel at the age of 17.

Mr Yousef left the Shin Bet in 2007 after he was accused of planning attacks against Israel, despite handing over crucial information for over ten years.

Mr Yitzak said of Mr Yousef - known as the "Green Prince" because the Hamas flag is green - said: "Mosab knows that as a handler I would have never wanted to let him go. He was too valuable to lose."

Mr Yousef immigrated to America where he claimed political asylum, but his application was denied over allegations that he had engaged in acts of terror.

When the Shin Bet refused to confirm that Mr Yousef had worked for them, Mr Yitzak broke his code of secrecy and revealed his own identity in order to testify for him.

He said: "Maybe one day they will look back and think they could have done things differently. Could you imagine if we still had him?"

Mr Yitzak was let go from the Shin Bet; one of the reasons was that he helped Mr Yousef. He said: "I was warned off and told not to testify for Mosab. I ignored it because I thought it was the right thing to do."

Mr Yitzak and Mr Yousef were in London last week on a fundraising trip with charity One Family UK, which supports victims of terror.

Mr Yousef is now estranged from his family after being labelled a traitor. He was often left to look after his family when his own father was locked up in Israeli prisons.

He said: "I was motivated by a sense of hate towards Israel at the beginning. I wanted revenge, but that hate soon evolved into curiosity and then an understanding of a different truth."

Now a Christian, he said witnessing Hamas supporters torturing Palestinians made him question his beliefs about who the enemy was.

He said: "I grew up in a very violent culture and a very violent society. I was often a victim of violence. People were killing and hating each other and I always found people made it too easy to blame everything on Israel.

"I came to realise Israel was not the only cause of our suffering. Palestinians had some problems that we didn't have the courage to face as a society.

Mr Yitzak said: "Do you know any Palestinians who have come from Hamas and talk for Israel the way he does?

"He is still very valuable but it doesn't mean Israel understands that… Israelis need to learn how to treat people better and say thank you. He worked for Israel for more than 10 years. What is his legal situation? Should he get a pension? He saved countless lives, he has had a letter from the Israeli parliament but I certainly think we should do more."