Maxime Bernier has resigned as foreign affairs minister, after he acknowledged leaving sensitive government documents out in the open -- apparently at his former girlfriend's home.

Sources told CTV News the documents included classified information for last April's NATO summit in Romania. One sensitive document contained details about NATO's military strategy in Afghanistan.

Bernier learned that he had left behind the documents Sunday night, but didn't tell Prime Minister Stephen Harper until Monday afternoon. He then resigned from his cabinet post.

"Mr. Bernier has learned and informed me that he left classified government documents in a non-secure location," said Harper. "This is a serious error and the minister has accepted his responsibility."

Trade Minister David Emerson will take over the post on an interim basis.

In Bernier's resignation letter to the prime minister, he wrote that "the security breach that occurred was my fault and my fault alone and I take full responsibility for my actions."

Harper said Bernier's resignation has nothing to do with his former girlfriend Julie Couillard, who has been linked with Hells Angels members.

"Let me be very clear: this is not to do with the minister's life or the life of a private citizen, 99 per cent of which I think is completely off bounds," said Harper.

However, Couillard told Quebec broadcaster TVA that the sensitive document had been left at her home in mid-April.

"Maxime came to my house and the document in question was left at my house," she said. "For now, what I can tell you is that the document made me feel very uncomfortable. I was referred to a top lawyer in that field, who told me ... what was the proper legal procedure (to return it)."

She added that she did not read the document, but that it was clearly addressed to Bernier.

"I was panicked by the fact that I had it," she said.

Opposition parties reacted quickly to the resignation, saying it was necessary but also pointing out there are still many unanswered questions that need to be addressed.

"The issue has now come to a breaking point," Liberal House Leader Ralph Goodale told reporters. "The prime minster has been very dismissive of this situation for weeks now, despite the questions -- serious, respectful questions that were put in the House of Commons -- (that) he simply refused to answer."

Goodale pointed to how dismissive Harper was earlier Monday afternoon of the situation. "Obviously now with events this evening the prime minister has been proven to be incorrect -- and that raises some questions about his judgement..."

NDP Leader Jack Layton called the Couillard incident "the straw that broke the camel's back."

"When the prime minister comes and just simply says there were some documents put aside in a way that may have been accessible to other people, everybody wants to know what the heck we're talking about."

Goodale pointed to a few major issues that need to be addressed, including details of the alleged security breach, and what plan the Tory government has to restore the credibility of the Department of Foreign Affairs, "which has obviously been damaged by the bumbling and bungling over the course number of weeks."

More questions

Earlier Monday, Liberal MP Michael Ignatieff had said the government should examine the role of Couillard in a company that bid on airport security.

Bernier first faced criticism after it was reported that Couillard had been involved with Hells Angels members, and the press has also seized on more personal details -- such as what Couillard wore to certain political events.

When Bernier was sworn in as the foreign affairs minister, Couillard arrived at the event in a revealing dress with a low neck-line, but she told TVA Tuesday it had been Bernier's idea.

"I didn't pick the dress, Maxime did," she said.

Ignatieff said the real issue is whether there were any links between organized crime and airport security, or the possibility of improper bidding for such security.

"I make no allegations here. I don't know the facts, and that's the point. I want someone in government to establish the facts and establish if there is -- or is not -- a link with a minister of the Crown," Ignatieff told reporters Monday.

"I don't care about her skirts, I don't care about her cleavage, I don't care about her past. I don't care about any of it -- it's none of my business, quite rightly."

Harper has repeatedly brushed off concerns about Couillard.

"I have no intention to comment on a minister's former girlfriend," Harper told reporters Monday at a joint news conference in Ottawa with Ukraine's President Viktor Yushchenko. "I don't take this subject seriously."

The new question came in light of a report in Montreal's Le Devoir newspaper on Monday that Julie Couillard, who had been involved until recently with Bernier, had been involved in a high-tech firm that had bid on airport security contracts. Her partner in that venture had biker connections.

The newspaper quoted a source as saying that in 2005, she had received documents that involved airport security.

Earlier reports have revealed that Couillard had other personal links to people involved in the biker underworld.

"There is a public security question when there is a possible link between airport security and organized crime," Ignatieff told the House of Commons' question period on Monday.

"If there is no link, all they have to do is stand up and say so, and we will leave the matter aside. But until we get an answer, we will ask the question."

Conservative House Leader Peter Van Loan said any security breach would have taken place under a Liberal government.

Couillard accompanied Bernier to his swearing-in ceremony as foreign affairs minister in August 2007. The first-term Quebec MP had been serving as industry minister before Harper shuffled his cabinet. She also travelled with the minister on some sensitive foreign trips.

A private matter

The Conservative government has maintained that Bernier's and Couillard's relationship is a private matter and that no threat to national security ever existed.

"The Liberal party is doing its best to turn question period into the Jerry Springer show," Van Loan said, referring to the once hugely-popular downmarket talk show.

The government has never addressed the question of whether a security check was done on Couillard. Harper again refused to address the question on Monday.

Couillard was to appear tonight on Quebec's TVA French-language network.

Bernier has been in hot water over other issues.

Last week, he promised the use of a Canadian Forces C-17 heavy-lift aircraft to take helicopters to Thailand for use in Burma cyclone relief. However, none of the four aircraft were available, and the government was forced to spend almost $1 million to rent an aircraft to fulfil the promise.

In April, Bernier came under heavy criticism near the end of a trip to Afghanistan after he told reporters during a scrum that Asadullah Khalid, the governor of Kandahar province, should be replaced.

With files from The Canadian Press and CTV's Robert Fife