Israel is to free 770 Israeli and Palestinian prisoners, including a founder of the Islamist movement Hamas, due to overcrowding, a prison services spokeswoman told AFP on Thursday.
Sivan Weizman said all of the prisoners, among them 200 Palestinians, had almost completed their sentences.
The prisoners, who were expected to be freed today, include Hassan Yusef, a West Bank leader of the Hamas movement, who had six weeks remaining of a six-year term for "membership of a terrorist organisation."
"Hassan Yusef is being freed today," Weizman said.
Last March, Yusef disowned his son after he admitted to having spied on Hamas for Israel and playing a major role in the arrest of senior militants.
Mosab Hassan Yusef, 32, has said he was a top informer for Israel's Shin Bet internal security agency, known by the codename "The Green Prince."
He converted to Christianity 10 years ago and now lives in California. He reportedly worked for the Shin Bet at the height of the Palestinian intifada, or uprising, that began in 2000, when Hamas carried out dozens of suicide bombings in Israel and Israel waged an all-out war on the group.
Hamas has said the elder Yusef worked entirely in the public political wing of the movement. He was elected to the Palestinian parliament in 2006, while in Israeli custody.
Deputy Palestinian prisoners minister Ziad Abu Ain told AFP on Thursday that the Palestinians were expecting the release of "around 70 prisoners who have finished their terms of Israeli jails" but had no additional details.
According to figures released by Israeli rights group B'tselem in April, there are 5,380 Palestinians held in Israeli jails, 217 of whom were under 18.