The Israel Prison Service (IPS) said he was found unconscious early on Tuesday. He was taken to hospital but attempts to resuscitate him failed.

The IPS said he had refused to undergo medical tests and get treatment.

After the death was announced, militants fired at least three rockets from the Gaza Strip, with no one hurt.

Islamic Jihad, which is based in the Gaza Strip, had earlier warned Israel would "pay a heavy price" should he die in jail.

Khader Adnan, 44, had been charged with belonging to a terrorist group and incitement and had been due to stand trial this month.

Adnan was from Jenin in the occupied West Bank, and had been in and out of Israeli detention over the past two decades.

He had been on hunger strike five times.

Before his death, his wife Randa Mousa said he was "refusing any support, refusing medical examinations, he is in a cell with very difficult detention conditions".

The fate of Palestinian prisoners in Israel is a top issue for the Palestinians, who hold Israel responsible for their well-being.