Senior Labor Party officials on Thursday sent a letter to party head Ehud Barak insisting that he unequivocally rule out joining a coalition that includes right-wing party Yisrael Beiteinu and its leader Avigdor Lieberman.

The letter was signed by Education Minister Yuli Tamir, along with Kibbutz Movement Secretary-General Ze'ev Shor and well-known Labor activists Luba Eliav and Professor Danny Gotwin.

"If you do not make a very clear and sharp statement that the Labor Party will not sit in any government with Lieberman, our ability as a party to make a moral statement and to fight for our way and our voters will be dealt a fatal blow," the letter read.

These statements come after other Labor MKs, including Shelly Yachimovich and Ophir Pines-Paz and Ministers Shalom Simhon and Benjamin Ben-Eliezer, put pressure on Barak to rule out a Labor-Yisrael Beiteinu partnership.

Barak has not specifically ruled out teaming up with Yisrael Beiteinu, and has only said, "Lieberman is not my cup of tea but we will not sit in any government whose guidelines are not compatible with our positions."

Barak said it would not be right for him to discuss possible coalitions at this time. "Any further reference to Lieberman would only boost him more. Say I form a government with all our friends and he wants to join on the basis of our principles - why not?" Barak said.

Yachimovich, one of Barak's staunch supporters, blasted him for this remark.

"I expect Barak to muster the political and moral strength required to state clearly that we will not be partners with Lieberman. Failing to do so turns Labor into a wanton slut up for grabs," she said.

"Ruling out a partnership with Netanyahu is not wise, because it implies support for Livni, while there is no reason to give it to her. But Lieberman is a dark, dangerous phenomenon. This is a person who brings out the darkest urges of part of the Israeli public. His slogan endangers democracy, and he is the moral red line which we must not cross. He is Kahane to all intents and purposes," she said.