The measure was one step beyond a previous government recommendation to self-isolate, and comes amid reports that some were ignoring the guidance.

"Earlier this month, we asked all travelers to self-isolate when they returned to Canada," Health Minister Patty Hajdu tweeted.

Earlier this month, we asked all travellers to self-isolate when they returned to Canada. Today, we are making this isolation mandatory under the Quarantine Act to better protect our most vulnerable. pic.twitter.com/bKLdKh4mEu

— Patty Hajdu (@PattyHajdu) March 25, 2020

"Today, we are making this isolation mandatory under the Quarantine Act to better protect our most vulnerable."

Deputy Prime Minister Chrystia Freeland told a news conference that self-isolation would be a "legal obligation" across the country as of midnight (0400 GMT Thursday). 

Under the federal law, which was made tougher in 2005 after the SARS crisis, those who disobey quarantine requests face fines of up to Can$1 million (US$700,000) or up to three years in jail.

Read also: Canada-US border likely to be closed by Saturday: Trudeau

The health minister can also designate specific quarantine zones.

In Newfoundland, a 53-year-old woman became the first person in Canada to be arrested for violating an order to self-isolate for 14 days after returning from a trip, under a provincial public health emergency law, police told AFP.

The Royal Newfoundland Constabulary responded to a tip about the woman spending time "in a public space", Constable James Cadigan said.

"Public health officials have been loud and clear: this virus could devastate our community," he said. 

The woman was to appear in court via videolink on Wednesday. She faces a possible fine and up to six months in prison under the Newfoundland law.