Canadian Prime Minister Justin Trudeau has apologised for wearing "brownface" make up at a private school gala nearly two decades ago.

The 2001 yearbook picture obtained by Time Magazine shows Mr Trudeau with skin-darkening make-up on his face and hands at the West Point Grey Academy.

Addressing the image, Mr Trudeau said he "deeply regretted" his actions and "should have known better".

The prime minister is battling for re-election on 21 October.

Mr Trudeau, son of the late former Prime Minister Pierre Trudeau, used to teach at the Vancouver private school.

The image is politically embarrassing for the prime minister because he has made progressive policies a signature issue.

Speaking to journalists after the Time article was published, Mr Trudeau said he had dressed up in the photo in an Aladdin costume at an Arabian Nights-themed gala.

"I take responsibility for my decision to do that. I shouldn't have done it.

"I should have known better. It was something that I didn't think was racist at the time, but now I recognise it was something racist to do and I am deeply sorry."

When asked if there had been other occasions, Mr Trudeau told reporters he had also worn make-up when he was a student performing at a talent show in high school.

An image from that incident has since been posted to Twitter.