TORONTO -- Ontario's Progressive Conservatives won a provincial byelection Thursday in northeast Toronto, but it may have come at a cost.

City councillor Raymond Cho won the Scarborough-Rouge River riding, defeating the Liberals in a seat they have held since it was created in 1999. It's the third byelection in a row the Tories have won since Patrick Brown became leader last year and a breakthrough for the party in Toronto, where they hold no other seats.

Brown said the victories herald a PC majority in the next provincial election.

"There is not any riding in the province of Ontario that Kathleen Wynne's Liberal party can take for granted," he said.

But the byelection race was dominated in the last week by a Tory flip-flop on sex education. A letter distributed under Brown's name promised that a PC government would "scrap" updates to the sex-ed curriculum.

It would have been a popular promise, Brown acknowledged, saying there was deep opposition to the curriculum in that riding.

But Brown disavowed the letter days later, saying he didn't know about it and actually won't scrap the curriculum despite what he calls a lack of parental consultation.

The Liberals seized on the chance to brand Brown a flip-flopper. But they also say questions remain about where the letter originated, whether it really was with the local campaign as Brown has implied.

Doug Ford, the campaign co-manager, said he did not write the letter.

Brown downplayed its significance, saying it "barely" got distributed. He still refused to say who wrote it, but said after the byelection he will "make sure that we have a better organization going forward."

"The lesson for me is to continue to focus on the fundamentals: on hydro, on jobs, on healthcare, to focus on the core issues that Ontarians care about," he said.

"I have no interest in wading into social issues."

The Liberals have also questioned why Brown only retracted the sex-ed opposition in English media, when the letter was distributed in other languages spoken widely in the riding. They suggested the reversal message did not reach as many voters as the original one.

Ford estimated only five or 10 per cent of people in the riding pay attention to the mainstream news.

Editorials from both left- and right-leaning media outlets roundly criticized Brown, saying the incident either shows he can't control his team, he can't pick a lane on a relatively straightforward issue or was just telling voters what they wanted to hear.

The controversy comes at a time when Brown is trying to rebuild a party after successive electoral losses and take it in a new, modern direction and sex-ed opposition did not fit with that message.

The curriculum was updated last year for the first time since 1998, but complaints from some parents ranged from not being consulted enough to the lessons being age inappropriate to anger over mentions of same-sex relationships, gender identities and masturbation.

Opposing the curriculum could hurt Brown provincially, though it would have played well in the riding, said Chris Cochrane, an associate professor of political science at the University of Toronto Scarborough campus.

Scarborough-Rouge River was a long-held Liberal riding, but Premier Kathleen Wynne's popularity has been sinking, mid-way through her mandate. Her plan to privatize Hydro One has not been received well and voters are upset about rising hydro bills.

Wynne released a statement saying the byelection result gives her "cause for reflection."

"We heard at the door that hydro rates are increasingly challenging for people," she wrote. "I understand, as do my ministers, that the government needs to focus on helping people with their everyday expenses."

Liberal candidate Piragal Thiru had about 29 per cent of the vote to Cho's 39 per cent. NDP candidate Neethan Shan trailed with about 27 per cent.

The byelection was triggered by the sudden resignation of Liberal Bas Balkissoon, who had held the riding since 2005.