By Dan Williams

JERUSALEM (Reuters) - Avigdor Lieberman formally tendered his resignation as Israel's foreign minister on Sunday and said he would focus on campaigning for next month's election on a joint ticket with Prime Minister Benjamin Netanyahu.

Although Netanyahu's rightist party alliance is expected to easily win the January 22 ballot, an opinion poll showed a dip in electoral support after Israeli prosecutors said on Thursday they would charge Lieberman with breach of trust.

Lieberman denies any wrongdoing in the case, which involved an Israeli diplomat who was promoted after leaking information to him about a police investigation into his activities.

He told reporters on Sunday he hopes for a quick trial and acquittal that would allow his return to high office.

"I am parting temporarily," Lieberman said. "The advantage is that it will leave me more time for activity ahead of the election. I intend to increase my involvement in electioneering, and I have no doubt what the outcome will be."

Lieberman heads the far-right Israel Beiteinu party, which is running jointly with Netanyahu's conservative Likud. Lieberman is number two on the slate, dubbed Likud Beiteinu.

A poll on the Israeli news site Walla on Friday predicted Likud Beiteinu would take 35 of parliament's 120 seats, down from 38 a day earlier.

Lieberman, a tough-talking immigrant from the former Soviet Union, said he wanted to prove his innocence in court. It is highly unlikely a trial could be concluded in the time left before the election and the formation of the next government, and some commentators have said he might seek a plea bargain.

Speaking on Sunday, Lieberman did not rule that out. "I very, very much hope this process will be a quick," he said.

Israel's Justice Ministry would not comment on the likely scheduling of Lieberman's case and the possibility of a plea bargain. But ministry spokesman Moshe Cohen said prosecutors would not object to speeding up proceedings, if that were requested by Lieberman.

Lieberman's resignation takes effect on Tuesday, after which the foreign portfolio will be assumed by Netanyahu, who has often sidelined Lieberman when it came to key relationships involving Washington and other Western allies.

(Editing by Sonya Hepinstall)