OTTAWA (AFP) - Canada's prime minister on Monday defended a proposed new law against prostitution, calling the sex trade inherently "bad and harmful".

Prostitution is "unacceptable to Canadians and unacceptable to our government", Prime Minister Stephen Harper said.

He said "the activities around prostitution are illegal because they are bad and harmful for women and for society more broadly.

"They are not harmful because they are illegal. They are illegal because they are harmful."

The Supreme Court last December struck down key provisions of the original law that effectively criminalised prostitution, saying that they endangered prostitutes.

The legal challenge was brought by three sex workers who argued that Canada's restrictions on prostitution - criminalising keeping a brothel, living off prostitution or soliciting sex in public - put their safety at risk.

The three Toronto women - Terri-Jean Bedford, Amy Lebovitch and Valerie Scott - argued that prohibiting brothels, for example, endangered prostitutes by forcing them to seek customers on street corners.

The law, they said, had also prevented them from taking safety measures such as hiring security guards or screening potential clients in an effort to protect themselves from violence.

They called for the right to open brothels to provide a safer environment for prostitutes.

A lower court found the measures, aimed largely at curbing nuisance crimes linked to prostitution, to be "arbitrary, overbroad or grossly disproportionate", and indeed put sex workers at risk.

The top court agreed, saying the curbs infringe on prostitutes' "constitutional right to security of the person".

The new law being considered by Parliament switches the focus of criminal charges from sellers of sex to potential buyers, and prohibits advertising sexual services.