Tzipi Livni, Israel's chief peace negotiator, has expressed doubts over the ability of Mahmoud Abbas, the Palestinian leader, to deliver an agreement even while insisting on the need for a historic deal to end the Middle East conflict.

Addressing foreign diplomats and journalists, Mrs Livni - who also serves as justice minister in Benjamin Netanyahu's cabinet - likened any agreement with Mr Abbas to a signed cheque on an empty bank account because he does not control the Gaza Strip, which is run by the Islamist Hamas movement.

She also acknowledged doubts over Mr Abbas' legitimacy in the West Bank - which his Fatah movement controls - because he has not held elections for several years.

Both the West Bank and Gaza are envisaged as part of a future Palestinian state.

"A legitimate question is with whom we are going to sign an agreement," Mrs Livni said in a speech at Jerusalem's landmark King David Hotel. "In the past, we had ties with those who wanted to make peace with Israel but couldn't deliver. And sometimes we had those who didn't want to make peace with Israel but they were strong enough to deliver. Now we have both.

"The Gaza Strip is being controlled by Hamas. They are not part of the peace. They are not willing.....to accept Israel's right to exist, to renounce terrorism and to accept all the agreements between Israel and the Palestinians.

"And on the other side, we have Abu Mazen [Mr Abaas], for many years without elections on the Palestinian side, without any possibility to change anything in Gaza, even if he wants to. So what's the use of signing an agreement?

"So in a way, I am going to sign an agreement, hopefully, with somebody who has signature rights on a bank account which is empty. It's completely empty in Gaza. But I need a signature. And officially he is the one who can sign."

The only solution was a "dual strategy" of reaching an agreement combined with an international effort to isolate and delegitimise Hamas, Mrs Livni said.

Mr Abbas has sanctioned "reconciliation" talks with Hamas after it fell out with Fatah following the Islamist group's violent takeover of Gaza in 2007.

Mrs Livni, a former Israeli foreign minister, urged the international community to order Mr Abbas abandon such moves because they "make Hamas stronger".

The comments reflect widespread Israeli doubts about Mr Abbas' credentials as a "partner for peace" - even as Mr Netanyahu, the prime minister, publicly proclaims readiness to restart negotiations.

Shimon Peres, Israel's president, addressed the Palestinian leader directly at a World Economic Forum meeting in Jordan in Sunday, telling him: "You are our partner and we are yours."

That contradicted comments by Yair Lapid, the finance minister, who in an interview last week called Mr Abbas "one of the founding fathers of the concept of Palestinian victimhood" and said he was not a true partner.

Mrs Livni, who on Monday met John Kerry, the US secretary of state, in Amman on the latest round of his efforts to re-start peace talks, said a two-state solution was necessary to keep Israel both Jewish and democratic while preserving its international legitimacy. But she admitted that some members of Mr Netanyahu's government were ideologically opposed to a peace deal.