Supporters of Mr Barghouti - who the Israelis say masterminded the deaths of dozens of Israelis - say he has suffered so-called "position torture" and sleep deprivation since being arrested by Israeli soldiers on April 15.

He has yet to be charged with any crime.

Mr Barghouti, the leader of Tanzim, the armed wing of Yasser Arafat's Fatah movement, is said to be the leader of the militants who have carried out shootings and bombings.

The Israelis also accuse him of being the founder of the Al-Aqsa Martyrs' Brigade, which has claimed responsibility for a large number of suicide bombings.

Mr Barghouti, who was jailed by Israel for six years in the 80s for membership of a Fatah faction, denies the allegations and says he is a political leader resisting Israeli occupation.

An Israeli government official emphatically denied claims of torture, saying that Barghouti is being "treated with kid gloves" because of his high profile. He added that if he was not sleeping,it was "because his conscience was keeping him awake".

Israeli government sources said yesterday that any decision on whether to prosecute Mr Barghouti would have to be made by cabinet, despite the fact that the constitution separates the executive and the legislature. Israel is likely to request that his period of detention be extended when he appears in court next week.

The allegations of mistreatment were repeated yesterday by Khalid Shkirat, a lawyer and the director general of LAW, a Palestinian human rights group advising Bargh outi. He said that Mr Barghouti told him he had been subjected to shabeh, a form of constraining torture, where the hands and legs are shackled to a small chair angled to slant forward, so that the victim cannot sit in a stable position.

Mr Shkirat said yesterday that Mr Barghouti had complained of pains in his back and hands caused by shabeh and had said he was being kept awake for periods of 20 hours at a time.

He has also alleged that his interrogators have made threats to kill him and his son, who is also in an Israeli jail.

"I have seen Marwan six times since his arrest," Mr Shkirat said yesterday. "He has lost an awful lot of weight and looks exhausted. He is still wearing the same track suit that he was given on his arrest.

"He has asked for underwear but has not been given any. He also told me he was allowed a two-minute shower once a week.'

Asked about the confession of involvement in terrorism that Israeli papers say Mr Barghouti made, Mr Shkirat said: "Marwan knew nothing about these claims. He says he has made no confession. He believes these claims were being made to undermine the confidence of Palestinians in their leaders.'

The use of sleep-deprivation, shabeh and violent shaking of hooded prisoners during interrogations was outlawed by the Israeli high court in 1999. Following that ruling, which makes those using torture liable to imprisonment and evidence obtained in such a way inadmissible, claims of torture declined. They have risen again since the intifada began in 2000.

"After the court ruling in 1999 there were virtually no reports of torture," Hannah Freidman, executive director of the Jerusalem-based Public Committee Against Torture in Israel, said. "When the intifada started in October 2000 it seemed to increase rapidly.

"We have collected many affidavits alleging torture and I have spoken to two of Marwan Barghouti's lawyers who have both reported how he has been interrogated for 20 hours at a time, shackled in a small chair."

Mr Barghouti was arrested by the Israeli army while leaving a safe house in Ramallah where he had been hiding during Operation Defensive Shield. He is currently being held at the Petah Tikva detention centre.

Allegations of Mr Barghouti's mistreatment while under interrogation by the Shin Bet security service follow claims by Israel that Barghouti was the key link between the Palestinian Authority chairman, Yasser Arafat, and terrorists involved in attacks on Israelis.

Israel has released documents that it claims prove that Mr Barghouti channelled funds between Mr Arafat and wanted "terrorists".