NDP Leader Jack Layton, with his wife NDP MP Olivia Chow, announces he has been diagnosed with prostate cancer. (Darren Calabrese/Canadian Press)

NDP Leader Jack Layton announced on Friday that he has prostate cancer but said he will not step aside as leader as he battles the disease.

"This year, more than 25,000 Canadian men are going to be diagnosed with treatable prostate cancer, and I recently learned that I'm one of them," Layton, 59, said from his Toronto riding. "It's the same kind of cancer that my dad was diagnosed with 17 years ago.

"He, like the overwhelming majority of Canadian men with prostate cancer, fought it and won. His treatments were successful, and I intend to bring to this battle the same sense of determination and optimism that he did.

"Like my dad, I'm a fighter, and I'm going to beat this."

The politician said his treatment plan is underway and that he's "feeling good."

"I want to ensure that my constituents know I will be carrying on as a member of Parliament for Toronto-Danforth and as leader of the New Democratic Party of Canada."

Layton's wife, NDP MP Olivia Chow, battled thyroid cancer a few years ago.

"She won her battle, and I'm going to win mine as well," Layton told media and supporters at his riding office.

Prime Minister Stephen Harper said he spoke with Layton earlier and wished him all the best in facing his health challenges.

"You know, it's a funny business. You do battle with a guy for many, many years, but at the same time … you often develop … very close relationships with some of your counterparts. And Laureen and I certainly consider Jack and Olivia friends," Harper said.

"As I told him, I know many people, as does he, who've faced this challenge and have overcome it, and so we'll be pulling for him. Our thoughts and prayers will be with him, and I look forward to doing political battle with him for many years to come."

Liberal Leader Michael Ignatieff said Canadians stand behind Layton in his fight against cancer.

"We all know how combative Jack is, and we know that he will face this challenge with his usual determination," Ignatieff said.

"While we often find ourselves lined up on different sides of the issues, we are always united in our desire to make Canada a better country and united in our respect for each other. Today, we are united in our support for one of our fellow MPs, who has contributed greatly to his country."

Bloc Québécois Leader Gilles Duceppe said the energy and determination Layton shows in his work in Ottawa will help hasten his recovery.

NDP deputy leader Thomas Mulcair said while Layton might lighten his workload, he'll still be working harder than most.

"Jack saying that he's going to pare back his schedule a little bit simply means that he's going to start working at about the same pace as any other party leader or politician because he works at a frenetic pace," Mulcair said. "He's a workhorse."

The Canadian Cancer Society estimates that in 2009, 25,500 new cases of prostate cancer were diagnosed and 4,400 Canadian men died from the disease.

Risk increases with age

Prostate cancer is the most commonly diagnosed form of cancer in the country, with lung cancer second and breast cancer third.

The risk of developing prostate cancer increases with age. More than 70 per cent of all prostate cancers are diagnosed in men older than 65.

Family history is another indicator. If a father or brother had prostate cancer, a man is up to 11 times more likely to develop the disease.

Depending on how far the disease has progressed, treatment can include hormonal therapy, surgery, radiation, chemotherapy or "watchful waiting."

Layton became leader of the federal NDP in 2003, but he was not elected to the House of Commons until the 2004 federal election.

In the 2008 election, his party gained eight seats, raising its total to 37.

With files from The Canadian Press