Beleaguered Frankston Liberal MP Geoff Shaw has been forced to apologise after uttering an obscenity and making an obscene gesture in State Parliament.

The Opposition accused Shaw of calling Labor MPs 'wankers' before making a derogatory hand gesture during a parliamentary sitting this morning.

The accusations come as MPs prepare to vote on whether Mr Shaw should face a parliamentary privileges committee over claims he misused his taxpayer-funded ministerial car.

Labor leader Daniel Andrews branded Shaw's behaviour obscene, and called for his immediate exit from the Liberal party.

"This is the last straw," he said.

"Geoff Shaw is an embarrassment."