Julie Couillard had been officially designated Mr Bernier's "spouse"
Canada's foreign minister, Maxime Bernier, has resigned after he admitted leaving classified documents in what he called "a non-secure location".
His former girlfriend, Julie Couillard, told Canadian media the papers had been left at her flat in Montreal in April.
Canadian Prime Minister Stephen Harper said it was a serious security breach.
Mr Bernier had been under pressure to quit after it emerged that Ms Couillard had previous relationships with men connected to organised crime.
"Minister Bernier has learned and informed me that he left classified government documents in a non-secure location," said Mr Harper.
"This is a serious error. (He) has accepted his responsibilities in offering to resign."
The announcement came hours before an interview with Ms Couillard was shown on Canadian television.
She said that Mr Bernier had come to her flat in April and forgotten a document there.
"He came to my place, then he left, and the document remained with me," she told the French-language TVA network.
It has not been revealed what the document contained, nor whether any other party saw it.
Ms Couillard said she returned it to the government via a lawyer.
Ms Couillard, 38, who had attended Mr Bernier's 2007 swearing-in as foreign minister, had officially been designated as his "spouse" so she could join him on official trips overseas.
It only recently emerged publicly that Ms Couillard had been romantically involved with two men who belonged to biker gangs that were involved in turf wars over drug dealing.
She has at no point been accused of any wrongdoing.
Ms Couillard and Mr Bernier broke up earlier this year.
Mr Harper denied the resignation had any connection to the "minister's private life".
"This is about one thing and that is a failure to uphold expected standards on government documents," he said.
"I have no intention to comment on a minister's former girlfriend," he added.
But a member of parliament for the opposition Liberals, Ralph Goodale, said the prime minister had "a lot of explaining to do".
Mr Bernier, who was appointed to the role last August, is the first minister to resign from Mr Harper's cabinet under a cloud since the Conservatives came to power in January 2006 elections.
He had earlier courted controversy when he publicly suggested that Afghan President Hamid Karzai replace the governor of Kandahar province, where Canada has 2,500 troops stationed.
Canada's International Trade Minister, David Emerson, is to serve as interim foreign affairs minister.