Mississauga South MPP Tim Peterson, brother of Ontario's former premier, has left the Liberal caucus to sit as an Independent and is expected to run for the Conservative party in the October election, CBC News has learned.

The news about the surprising political defection at Queen's Park came around noon Thursday.

A backbencher since he was first elected in the 2003 provincial election, Peterson's departure does not represent a huge loss for the party.

However, the Peterson family has a long history with the party — one brother, David Peterson, is a former Liberal premier and the other, Jim Peterson, is a former federal cabinet minister.

"It does sort of attack a core franchise of the Liberal party," CBC Radio reporter John McGrath said, describing Peterson's departure as the "equivalent of a cabinet minister crossing floor" because of the family's history.

Sources told CBC News that Peterson gave Premier Dalton McGuinty an ultimatum: assure him of a spot in cabinet before or after the upcoming provincial election or he would leave.

The Liberals apparently told him the province doesn't offer cabinet post guarantees.

Peterson's defection leaves the Liberals with 68 seats, while the Conservatives still hold 24 and the New Democrats have 10.

The next provincial election is scheduled for Oct. 10.