The main opposition party in Canada, stunned by its leader's serious illness, vowed on Wednesday to fight on and dismissed speculation that it could start to break up.

New Democratic Party leader Jack Layton, who took the left-leaning party to its best ever performance in the May 2 election, announced on Monday he was stepping down temporarily to deal with a second bout of cancer.

Layton, who looked gravely ill, vowed to return for the resumption of Parliament on Sept 19. The party relies heavily on his charisma and experience and some political observers predict the NDP could struggle without him.

"These are terrible circumstances, no doubt about that," NDP president Brian Topp told reporters. "(We) don't celebrate what is happening here. But I think we've got the bench strength in this team to step forward and handle it." .

The party is already making clear that Layton could be off work longer than planned.

"If Jack Layton needs more time to recover, no one in the party will tell him he can't take that time," said Topp, who spoke as NDP members of Parliament met in Ottawa to choose an interim leader.

Layton -- who was due to address the meeting by phone -- wants political newcomer Nycole Turmel to take the job and looks set to get his way. Party officials will officially announce the interim leader on Thursday.

Turmel, who is in her late 60s, would not be a candidate if Layton did not return and the party had to run a leadership race.

Senior party legislators, who said Layton looked to have successfully beaten off the prostate cancer that was diagnosed last year, acknowledged they were shocked by the news that he now has to fight a second cancer but said they remained upbeat.

"This party will pull together. Jack has to look after himself and us. We will look after the politics," said veteran parliamentarian Yvon Godin.

A wave of so-called "Laytonmania" helped the NDP elect enough members of Parliament on May 2 to become the main opposition party for the first time. The governing Conservatives won a majority in the House of Commons and will not face reelection until October 2015.