The leaders who recently merged their unions to form the largest industrial private sector union in Canada have announced they are both stepping down.

Ken Lewenza, the head of the Canadian Autoworkers Union, and Dave Coles, the president of the Communications, Energy and Paperworkers Union of Canada, have announced they are resigning.

The two men made the announcement at a news conference in Toronto on Thursday.

The two unions voted last year to merge and the leaders endorsed senior CAW official Jerry Dias to be the first president of the new combined union, to be called Unifor.

Of their endorsement, Dias said, "I accept their confidence level. The test will be at our convention in September."

"I’m not putting the cart before the horse, I’ll let the members decide who will be their next president. You can’t take the rank and file and the local union for granted," Dias said

Dias told CBC's Evan Solomon that the merger was about "being relevant, creating opportunities for young people, it's about getting involved in politics, because that has so much to do with our working lives.... It's about turning up the heat."

Lewenza said his last day will be Oct. 1, 2013. "I'll be an ambassador for Unifor, I'll be an activist," Lewenza said of his post-retirement plans.

Dias, a top lieutenant under Lewenza's tenure at the top, has worked his way up the CAW ranks during the last 35 years.

Lewenza said Dias "will be able to step in because of his experience. He’s been in the middle of the peaks and valleys. He’s been in the crises."

Lewenza said that at 59 years old he wasn't willing to commit to the work that needs to be done as Unifor merges and expands from its base. "This is not a one-year job, this is a three- or four-year job, minimum," for whoever heads up the new union.

Lewenza was elected the national president of the CAW in 2008, replacing Buzz Hargrove, and Coles has been president of the CEP since 2006.

"Kenny’s very dynamic and a great leader," Hargrove said of his successor. "The thing Kenny always cared about is what he leaves behind.

"He obviously came to the conclusion it was in the best interest of the union," Hargrove said Thursday.

The new union will represent more than 300,000 workers across roughly 20 sectors of the economy, primarily in manufacturing, communications and transportation. Unifor will be the country's biggest private-sector union.

Lewenza told CBC's Rosemary Barton on Power & Politics on Thursday that the new union presents another opportunity for renewal.

"The labour movement, like every other organization, has to look in the mirror from time to time and renew their commitment and renew their values and raise expectations — and that's what we're trying to do through this new union Unifor."

When asked whether he would consider running for any political party, Lewenza didn't offer a yes or no answer, saying instead that he's never planned his career.

During an earlier interview with CBC News, when he was working on merger plans, Lewenza indicated that leading a union is getting tougher.

"We got the privilege of representing the workers, they got the privilege of putting us in, putting us out," Lewenza said. "But at the end of the day, man, it's a bigger job today than it was 40 years ago."

Unifor will also represent some public-sector employees in the health, education and transit sectors.

With files from The Canadian Press