Mr Shahak handed in his resignation as Chief of Staff last June after a disagreement with Mr Netanyahu.

He said: "The decision I'm making is not easy and is motivated by a sense of responsibility which forced me to embark on a new path".

"A lot of people are expecting something new and if I can help, I'll do it."

Opinion polls give Mr Shahak a slight edge in the race against both Mr Netanyahu, of the right-wing Likud Party, and opposition Labour leader Ehud Barak.

Mr Shahak, 54, has already rejected Mr Barak's call to join his left-wing party. Labour leaders have called Mr Shahak's decision irresponsible, saying he risks splitting the peace camp.

Mr Shahak, a soldier for 36 years, told an Israeli newspaper he wants the number one or two position in a centrist party capable of ousting Mr Netanyahu.

He has also said he would be willing to be deputy to Dan Meridor, who has split from Likud.

Mr Meridor announced he would run for prime minister against Mr Netanyahu after the Israeli parliament voted for early elections last Tuesday. He said Mr Netanyahu lacked credibility.