Former Labour leaders and high-profile figures including authors Amos Oz and David Grossman are to launch a new movement to run jointly with Meretz in February's elections.
Their aim is to win 10-12 seats in the next Knesset and replace Labour as Israel's largest left-wing political camp.
Leading figures in the new grouping, to be announced today (Friday) include former Labour ministers Uzi Baram and Shlomo Ben-Ami, and former Knesset speaker Avraham Burg.
Others include several of Labour leader Ehud Barak's former allies, including his former chief-of-staff Tzali Reshef, Gilad Sher, who headed negotiations with the Palestinians during his premiership and Yossi Kuchik, former director-general of Mr Barak's office.
The initiative comes as a further blow to the Labour leader, who recent polls suggest stands to win only 10 seats in the coming elections, down from 19 in 2006. Sources within Meretz said that Mr Barak had personally phoned some of the new group's people to ask them to quit the initiative.
Support for Meretz has also been steadily falling. Its parliamentary presence fell from 12 MKs in 1992 to five MKs in 2006. Of these five, Ran Cohen and former chairman Yossi Beilin have recently resigned from politics.
Mr Burg, also a former Jewish Agency Chairman, said that Labour had lost its relevance and that the new initiative sought to offer a valid alternative.
"Whoever likes content will come to our group instead of finding a safe haven in the emptiness of Kadima," Mr Burg said. "It's the early stages of trying to rebalance the ideological and political map of Israel. In the last couple of years there was a feeling that between the Likud and the left there was a big void. Kadima is the party of those who have no opinion, and Labour disappeared. So who will answer the ultra-right neo-conservative Netanyahu position? Up until yesterday the answer was ‘nobody'. From now on we try to give an alternative political response."
Labour secretary-general Eitan Cabel said: "In the near future, before the general elections, we'll witness many such initiatives, but the Labour party has the deepest roots in Israeli society and it is Labour that leads and will continue to lead the Israeli peace camp."