Liberal Leader Stéphane Dion will not run a Liberal candidate against Green party Leader Elizabeth May in the next federal election, the first time the Liberals won't plan on having a full slate of candidates.

The leaders have agreed not to run a candidate in each other's riding. They will hold a joint news conference about their deal on Friday in Halifax.

May, who does not have a seat, has chosen to run in the Nova Scotia riding of Central Nova, held by Foreign Affairs Minister Peter MacKay. The riding is considered a longtime Conservative stronghold. In the last federal election, MacKay's winning margin was 3,300 votes.

CBC News has learned that leaders will say Friday that "out of our shared commitment to a greener Canada, we are not running candidates in each other's ridings."

CBC's chief political correspondent Keith Boag said the Liberals gain a credible ally on the environment, something Canadians continue to rank as one of the most important issues.

Boag said that May is widely respected for her judgment about climate change and that she will send a clear signal Friday that she thinks Dion would make a better prime minister on that issue than Harper.

Dion had hinted weeks ago that there was something worth trying to develop with May as observers continue to speculate about the possibility of a spring election.

"Madame May and I have conversations about how we may work together to be sure that this government will stop to do so much harm to our environment," Dion said on March 21.

Boag said the Liberals have always prided themselves on running a national slate of candidates.

"This is a pretty stunning thing for the Liberal party to do," he said.

He said the move could cause Dion some trouble within his caucus. He said Dion already spoke to some caucus members who said they are uneasy with the decision.

But Dion told his Nova Scotia members Thursday night that he's going ahead with the deal, Boag said, and they told the Liberal leader they will be at his side when he makes it public Friday morning.

The Green party received 4.5 per cent of the popular vote in the January 2006 election, and its candidate in MacKay's riding of Central Nova tallied less than two per cent of the vote.