Canadian Prime Minister Justin Trudeau wore “brownface” at a private school gala nearly two decades ago, a photo obtained by Time Magazine shows.
Addressing the image, Mr Trudeau said he “deeply regretted” his actions and “should have known better”.
The 2001 yearbook picture shows Mr Trudeau with skin-darkening make-up on his face and hands at the West Point Grey Academy in Vancouver.
The prime minister is battling for re-election on 21 October.
Mr Trudeau, who is the son of late former Prime Minister Pierre Trudeau, used to teach at the elite academy.
The image is politically embarrassing for the prime minister because he has made progressive policies a signature issue.
How did Trudeau respond?
Speaking to journalists after the Time article was published, Mr Trudeau said that he was dressed 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 that he also wore make-up while performing at a talent show in high school.
An image from that incident has since been posted to Twitter. A source confirmed to the BBC that it was from the incident Mr Trudeau referenced in the news conference.
The National Council of Canadian Muslims responded critically to the report.
Executive director Mustafa Farooq said: “Seeing the prime minister in brownface/blackface is deeply saddening. The wearing of blackface/brownface is reprehensible, and hearkens back to a history of racism and an Orientalist mythology which is unacceptable.
The council added that it recognised that “people can change and evolve over two decades”, but said Mr Trudeau must “immediately and unequivocally” apologise.
Later, the council issued a tweet thanking him for apologising promptly.
Andrew Scheer, leader of the opposition Conservatives, said the picture was racist in 2001 and racist now.
“What Canadians saw this evening is someone with a total lack of judgement and integrity and someone who is not fit to govern this country,” he said.
New Democratic Party leader Jagmeet Singh, a Sikh, said the image was “troubling” and “insulting”.
“Anytime we hear examples of brownface or blackface, it’s making a mockery of someone for what they live and what their lived experiences are,” Mr Singh told journalists on the campaign trail in Toronto.
The image was also criticised in a tweet by Green Party leader Elizabeth May.
Opinion polls indicate that October’s election will be a tough race for Mr Trudeau who is seeking a second term in office.
His campaign got off to a bad start after his plane was grounded by a scraped wing on the first day. A bus ferrying journalists collided with the wing of the Liberal party’s chartered Boeing last week.
It’s not the first time a politician has got into trouble in this way. Virginia governor Ralph Northam admitted he had worn blackface while dressing up as Michael Jackson.
However he denied claims that he had been in a racist photograph in his 1984 medical school yearbook.
Justin Trudeau: Canada PM in ‘brownface’ 2001 yearbook photo