The son of a jailed Hamas leader who converted to Christianity and moved to California has gone public to say that he spied for Israel.
Speaking before the release of a book about his life, Mosab Hassan Yousef made the assertion in an interview for Israel's Haaretz newspaper.
A former deputy head of Israel's Shin Bet intelligence service told BBC World Service he had been one of its agents.
But a Hamas leader dismissed the report as a slander on the Islamist group.
Mr Yousef, 32, is a son of Sheikh Hassan Yousef, a senior Hamas figure in the West Bank, who is currently serving a six-year prison sentence in an Israeli prison.
" He provided very important information like hundreds of others fighting against terror"
Former deputy leader of Shin Bet
Former deputy leader of Shin Bet
If the younger Yousef's revelations are true, and he did play a role in preventing Hamas attacks on Israel, it will be an embarrassment for the group, which prides itself on its tight discipline and shuns the Palestinian Authority because of its peace negotiations with Israel.
While the Yousef case dates back several years, there have been suggestions the group was betrayed more recently when Hamas figure Mahmoud al-Mabhouh was assassinated in Dubai on 20 January.
'Slander and lies'
Mosab Hassan Yousef converted to Christianity and moved to the US in 2007.
The book he co-wrote, Son of Hamas, is due to be published there shortly.
Referred to officially as Shabak in Hebrew or the Israel Security Agency in English
Charged with defending against terrorism, subversion and espionage
Interrogates suspected militants at special centres across Israel
Has been accused of using torture against Palestinians
"He provided very important information like hundreds of others fighting against terror," Gideon Ezra, formerly deputy leader of Shin Bet and now a member of the Knesset for the Kadima party, told BBC World Service.
Mr Ezra said the younger Yousef had been persuaded to spy for Israel while being held in prison himself.
Earlier, senior Hamas leader Ismail Radwan condemned Haaretz's report as "baseless slander" aimed at the elder Yousef.
"The Palestinian people have great confidence in Hamas and its struggle and they will not be fooled by this slander and these lies of the Israeli occupation," he told AFP news agency.
Haaretz journalist Avi Issacharoff, who wrote the original article, told the BBC Mr Yousef was not prepared to give any further media interviews as of Wednesday morning.
News of his religious conversion in 2008 shocked many Muslims in the Gaza Strip and he was condemned by some for his "apostasy".
Mosab Hassan Yousef was considered Shin Bet's most reliable source in the Hamas leadership, earning himself the nickname "the Green Prince" because of the colour of the group's flag and his pedigree as the son of one of the movement's founders, Haaretz writes.
"I wish I were in Gaza Now" Mosab Hassan Yousef
One of his Israeli "handlers" told the paper that he had saved many lives, with one of his insights "worth 1,000 hours of thought by top experts".
"The amazing thing is that none of his actions were done for money," the handler, named in the book as "Captain Loai", added.
Speaking to the newspaper by phone from California, Mr Yousef appeared to be still brimming with enthusiasm for Israel's fight against Hamas.
"I wish I were in Gaza now," he was quoted as saying. "I would put on an army uniform and join Israel's special forces in order to liberate [Israeli hostage] Gilad Shalit."
Gideon Ezra told the BBC that it was not easy for Israel to penetrate Hamas but it was "doing its best".
His country, he argued, had no choice but to recruit agents within Palestinian militant groups in order to avoid attacks, though he added that the security situation in the West Bank had improved under the Palestinian Authority from Israel's perspective.
Asked about his own experience of recruiting Palestinian agents, he said there were many motives for them to turn spy.
"It depends on each person," he added. "You can't do it through threats. If they don't do it willingly, you can't force them to be your informant."