A senior Hamas leader publicly disowned his son Monday, days after the young man announced he had secretly spied for Israel and helped authorities hunt down members of the Islamic militant group.

Hamas Web sites published a letter late Monday by Sheik Hassan Yousef that the militant group said was smuggled out of the Israeli prison where he is serving a six-year sentence.

In the letter, he said his family announced its "complete renunciation of the one who was once our eldest son, who is called Mosab." The father said he was sorry to take such a step but said he had no choice after his son "disbelieved in God...and collaborated with our enemies," he said.

The elder Yousef, who helped found the militant Islamic group two decades ago, was humiliated last year when his eldest son announced he had converted to Christianity. Then the son told an Israeli newspaper last week that he had helped Israeli intelligence foil militant attacks and hunt down Hamas leaders — including his father.

Mosab Yousef told the Haaretz daily said he spied for Israel for a decade before fleeing to California in 2007. Mosab Yousef is publishing a memoir, "Son of Hamas."

Monday's announcement means the family now considers their son to have never existed. He loses his inheritance and the family will never speak to him, or about him, again.

Mosab Yousef did not immediately respond to messages seeking comment on his family's decision.

Palestinians, much like Arab communities across the region, are typically deeply private and try to conceal damaging family fissures. Only in very rare cases do families publicly renounce their children — usually when a son or daughter has done something in public that the family considers deeply humiliating.

In his book, Mosab Yousef claims he was considered one of the Shin Bet security agency's most valuable assets and was dubbed "The Green Prince," a reference to his Hamas pedigree and the Islamists' signature green color.

The claims have exposed a new side of the Islamic militant's vulnerability, coming in the wake of the assassination of a top Hamas operative in Dubai in January.

Israel has not commented on Yousef's claims, or on widespread speculation that it carried out the Dubai assassination.