The article was based on extracts of a book, "Son of Hamas", co-written by Yousef, who converted to Christianity 10 years ago and now lives in California. The book is set to be published in the US next week.
The revelations, if true, are a further blow to Hamas, and its shadowy military wing. They follow accusations that an informer within the group aided the killers of a top Hamas operative in Dubai, an assassination widely blamed on Israel's Mossad but for which no evidence has been made public.
Hamas and Yousef's father denied he could have had any access to sensitive information.
However, the article said he was crucial in the arrests of Ibrahim Hamid, once the movement's military chief in the West Bank, and Abdullah Barghuti, the bomb maker behind an infamous 2001 suicide attack on a Sbarro restaurant in Jerusalem.
He was also said to have played a role in the arrest of Marwan Barghuti, a senior Fatah leader considered the architect of the 2000 intifada, or uprising. The two Barghutis are not closely related.
Yousef reportedly worked for the Shin Bet at the height of the uprising, when Hamas carried out dozens of deadly suicide bombings in Israel and Israel waged an all out war on the group, killing many of its top leaders.
Haaretz quoted Yousef's handler, identified only as Captain Loai, as saying that the informer was motivated by a desire to prevent attacks.
"The amazing thing is that none of his actions were done for money. He did things he believed in. He wanted to save lives," the paper quoted the handler as saying.
Sheikh Hassan Yousef -- a senior Hamas leader in the West Bank who was arrested in September 2005 and is still being held in an Israeli jail -- denied his son had access to Hamas secrets.
"Mosab was never an active member of Hamas in any capacity, either in the political or armed wing," he said in a Hamas statement issued in his name.
"From 1996, when he was 17 years old, he faced blackmail and pressure from Israeli intelligence and when he revealed his situation at that time the sons of the movement were warned about him," the statement said.
Hamas also said the elder Yousef worked entirely in the public political wing of the movement. He was elected to parliament in 2006.
Israeli analysts said the revelations were a deep blow for Hamas.
"For Hamas, this was one of the worst possible things that could have happened to them, the son of a Hamas founder betraying them," said Mordechai Kedar, a veteran of Israel's military intelligence and an expert on Arab affairs at Bar Ilan University.
However, he said the book could also cause problems for the Shin Bet, which has a long history of using Palestinian informants.
"Everything that is published about agents or the way they operate does not help," Kedar said.
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