TORONTO - Premier Dalton McGuinty's Liberal government suffered a public relations blow Thursday when backbencher Tim Peterson, the younger brother of former premier David Peterson, said he was quitting the party to run as a Conservative in October.

Losing a member from one of the first families of Ontario Liberal politics is not the kind of news the government needs just six months before an election, but party strategists insisted Peterson had been unhappy for years about being left out of cabinet.

First elected in 2003 in the riding of Mississauga South, Peterson, 59, joined Conservative Leader John Tory at a news conference at the legislature to announce his defection, blaming the government's failure to address issues in his riding west of Toronto.

"After much consideration, I decided to leave the Liberal caucus to sit as an Independent in this last session of the legislature prior to the election," Peterson said.

"After this session, I will join the caucus of the John Tory Progressive Conservative Party of Ontario, and will be their candidate in Mississauga South in the election on Oct. 10."

McGuinty said he wasn't going to let Peterson's move influence the government's agenda or re-election plans.

"I'm disappointed to learn of the news about Tim, but I have always said what we're doing here is bigger than any one individual," McGuinty said.

Peterson had been parliamentary assistant to Health Minister George Smitherman, who is also deputy premier and a close friend of the Peterson family.

Smitherman said he tried to reach out to Peterson on Wednesday before the local riding association learned of the plan, but only had a "ten-second telephone conversation" and didn't get a chance to talk him out of the decision.

"I know the other brothers better than I know Timmy, but I'm a Peterson guy, so I've always sought to try and be an ally of his," said Smitherman.

"I was a bit saddened that he didn't take the time to talk to anybody."

In fact, Peterson did not inform McGuinty of his decision to quit the party when the two met briefly Wednesday evening, even though he had already made his deal with the Conservatives.

"I learned this morning," McGuinty said. "I met with him late (Wednesday) afternoon, we had a meeting and . . . he certainly didn't make it clear that he had made a final decision."

Peterson said his first inclination was to quit politics, but after starting talks with Tory last December, he decided to switch parties and run as a Conservative.

He said he initially wanted to cross the floor but opted instead to bide his time until the next provincial vote.

"It's not about Mr. McGuinty's leadership; it's about the issues in Mississauga South," he said.

"I never expected to be in cabinet because I was not one of Dalton's chosen to run, but I did expect to . . . at least have my voice heard."

Peterson was the first Liberal to represent Mississauga-South since Confederation, but party insiders said Thursday they were confident they could hold the riding this fall, and predicted Peterson would come to regret his decision to cross the floor.

A former merchant banker, Peterson said Wednesday turned into an emotional night when he informed his riding association about the decision.

"This was very tough . . . some tears were shed all around," he said.

Peterson's older brother David, 63, was premier of Ontario from 1985 until 1990. Another brother, Jim, 65, has been a Liberal MP for the Toronto riding of Willowdale for more than 20 years and also served as a cabinet minister under former prime minister Paul Martin.

Peterson's defection leaves the Liberal government with 68 seats, while the Conservatives hold 24 and the New Democrats 10.

His decision follows that of another Liberal, Ottawa veteran Richard Patten, who announced earlier this month he would not seek re-election in part because he, too, was never promoted to cabinet.