Premier Dalton McGuinty says that while he's disappointed one of his MPPs has quit the Liberal caucus, he doesn't think the departure of one person will affect the performance of his government.

Tim Peterson, younger brother of former Liberal premier David Peterson, has decided to sit as an Independent and then run for the Progressive Conservatives in the next provincial election.

Sources told CBC News Thursday the 59-year-old MPP left because McGuinty would not assure him of a spot in cabinet before or after the election.

As he announced his decision, appearing with Conservative Leader John Tory, Peterson said he was leaving because of the government's failure to address the needs of constituents in his riding of Mississauga South.

Peterson was first elected to the riding in 2003.

Tory said Peterson never mentioned any desire to be in cabinet when they talked. Tory added that Peterson was not the only Liberal backbencher who complained to him.

Backbenchers frustrated, Tory says

"I think there's a great deal of dissatisfaction in the Liberal backbenches that has to do with the fact they're not listened to," Tory said, adding that backbenchers have difficulty raising local issues.

When asked why he never brought Peterson into his cabinet, McGuinty replied that there are many bright prospects in his party, but tough choices have to be made about who gets to sit at the table.

"This pursuit of ours is much bigger than any one individual. I've got a very strong team and we're not going to take our eye off the ball. It's not the kind of thing that you want to see happen, but it does happen. Our responsibility is to carry on and keep moving forward, and we'll do just that."

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.