Progressive Conservative Leader Patrick Brown is making a hard-right turn, promising to scrap Ontario's Liberal government's new sexual-education curriculum on the eve of a close-fought Toronto by-election.

In a letter addressed "Dear Parents" and dated this week, Mr. Brown pledged to kill the curriculum.

"I believe parents are the primary educators of their children. When it comes to sexual health education, parents should have a say on how much their children are taught, and at what age," Mr. Brown writes in the letter, a copy of which was obtained by The Globe and Mail on Friday. "Upon being elected, a PC government would scrap the ‎controversial changes to sex-ed introduced by Premier Kathleen Wynne."

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Mr. Brown did not release the letter to media until after its existence was uncovered by the Toronto Sun.

A spokeswoman said in an e-mail to The Globe that the letter was distributed to families in Scarborough.

Tamara Macgregor, a spokeswoman for Mr. Brown, said that the Conservatives would scrap the "controversial changes" and introduce new curriculum after consulting with parents.

"You'll see from the letter that the leader does believe in the importance of having sex ed in schools," she said.

The PC policy represents an abrupt about-face for Mr. Brown. Since winning the PC leadership in May, 2015, Mr. Brown has tried hard to present himself as a "modern conservative." Among other things, he has sworn off raising divisive social issues such as abortion and embraced carbon pricing to fight climate change.

This spring, he even told The Globe that he personally has no problem with the content of the sex-ed curriculum.

While Mr. Brown had a reputation as a social conservative during his time as a federal MP – he once voted to reopen the abortion debate, and won the provincial PC leadership by appealing to pro-life and anti-sex-ed elements within the party – he signalled that he had changed his views after his victory.

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But now, the PCs are locked in a tight-race in Scarborough-Rouge River, a Liberal bastion that the Tories badly want to win. And there is opposition to the sex-ed curriculum within some groups of new Canadians in the riding‎.

A PC source said the sex-ed issue has come up repeatedly with voters in Scarborough, even as it has died out in other parts of the province.

"When you talk about education at the doors here, it's the first thing people bring up," the senior Tory insider said.

Ms. Wynne declined to answer questions on Mr. Brown's letter while campaigning at a Tamil street festival in Scarborough Friday evening.

Education Minister Mitzie Hunter said it would be "irresponsible" to axe the curriculum changes, pointing out that the previous curriculum was 15 years out of date and a new version was necessary to keep Ontario in line with other provinces.

"Not in this century. It's an irresponsible thing to do," she said when asked about Mr. Brown's promise. "For the leader of an opposition party to insert that into a campaign, it's actually pretty deplorable. The responsibility that we have is to keep our kids safe and to make sure that they have the information that they need."

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The new health curriculum was introduced last fall, hailed by many in the education community as a necessary update to a curriculum that did not address the realities children face today. But a small, vocal segment of parents protested against the curriculum. Some pulled their children from public school in favour of homeschooling or private schools.

The controversial curriculum handles topics such as acceptance of same-sex couples and online safety, as well as teaching students the proper names for body parts, including genitalia, and that masturbation is normal.

The Liberals have accused opponents of the sex-ed curriculum ‎of homophobia.