Canada's main opposition New Democratic Party, already hobbled by the sickness of leader Jack Layton, hit more problems on Tuesday when it emerged his interim successor had belonged to a Quebec separatist party.

The news that Nycole Turmel had been a member of the Bloc Quebecois -- which is dedicated to Quebec's separation from Canada -- could damage the left-leaning NDP, which won enough seats in the May 2 federal election to become the official opposition for the first time.

The ruling Conservatives said the revelation showed the NDP was not up to the job of governing Canada.

NDP officials said Turmel -- appointed last week to replace Layton while he battles a second bout of cancer -- had joined the Bloc as a favor to a friend.

"She took a membership card to support a friend. She wasn't exactly a founder of the Bloc," said spokesman Karl Belanger.

The NDP crushed the Bloc and the Conservatives in Quebec in the May election, winning 59 of the province's 75 seats.

The Globe and Mail said Turmel had been a Bloc member from December 2006 to January 2011. The news will only add to the pressure on Turmel, 68, a former union leader, who was elected for the first time in May and has little experience of federal politics.

Prime Minister Stephen Harper, whose Conservatives won a majority in May, has long taken a hard line against what he calls the separatist threat in largely French-speaking Quebec.

"We all support our friends -- that doesn't mean we buy a membership for five years with a party that wants to break up Canada," said chief Harper spokesman Dimitri Soudas.

"This is yet another worrying example of the NDP not (being) up to the job of governing Canada," he said via Twitter.

The Globe reported Turmel had sent a letter to the Bloc in January 2011 to announce her resignation for personal reasons, saying her move had nothing to do with the party's policies.

A month later she announced she would run in the next election for the New Democrats. Belanger said the party had been aware of Turmel's Bloc membership.

During the election campaign, Turmel was criticized for supporting Bloc candidates in 2006. She said she backed the Bloc's social justice policies -- which are similar to those of the NDP -- and did not favor breaking up Canada.