TORONTO - Former Liberal Tim Peterson has officially joined the Progressive Conservative party, attending his first caucus meeting today.

Peterson, younger brother of former Premier David Peterson -- and a member of one of Ontario's most prominent Liberal families -- quit the government in late March.

Expressing frustration over local issues in his Mississauga-South riding, Peterson sat as an independent until the legislature prorogued Tuesday.

Opposition Leader John Tory had promised Peterson would not have to face a nomination battle in Mississauga-South if he switched from the Liberals to the Conservatives.

The two agreed he should not cross the floor while the legislature was sitting, and the house will not be back in session until after the Oct. 10 provincial election.

Peterson's defection leaves the Liberal government with 68 seats, while the Conservatives hold 25 and the New Democrats 10.

Tim Peterson was the first Liberal to represent Mississauga-South since Confederation, and Liberal insiders predict he will come to regret his decision to change parties.