LONDON • British Prime Minister Theresa May piled pressure on her former foreign secretary and potential rival for leadership of the Conservative Party, Mr Boris Johnson, to apologise after he said the Muslim burqa, worn to conceal a woman's face and body, made wearers look like bank robbers and letter boxes.

Mr Johnson used his weekly column in the Telegraph newspaper on Monday to argue against a ban on the burqa, as implemented by Denmark, France and Austria, saying people should be free to choose their own dress.

At the same time, he mocked the garment, saying it was "absolutely ridiculous that people should choose to go around looking like letterboxes", and that face coverings made the wearer resemble a "bank robber".

As his remarks created a furore, Mrs May told reporters on Tuesday: "Women should have the freedom to choose how they dress... It's very clear that the language that Boris used has offended people."

She said she agreed with the chairman of her Conservative Party, Mr Brandon Lewis, who had asked Mr Johnson to apologise although he did not say if there would be any sanctions if the request was ignored.

Mr Johnson, who has a reputation for causing controversy and quit Mrs May's Cabinet last month in protest to her Brexit plan, has so far refused to back down.

"It is ridiculous that these views are being attacked - we must not fall into the trap of shutting down the debate on difficult issues," a source close to Mr Johnson told reporters. "We have to call it out. If we fail to speak up for liberal values, then we are simply yielding ground to reactionaries and extremists."

Mr Johnson has been touted as a potential prime minister by those in the Conservative Party who want a harder departure from the European Union. But others saw his comments as an example of why he would be a poor candidate.

Conservative peer Sayeeda Warsi, a former party chairman, accused Mr Johnson of adopting the "dog-whistle" tactics of right-wing firebrand Steve Bannon, US President Donald Trump's former top aide. Mr Johnson has been in direct communication with Mr Bannon in recent months, according to media reports.

Ms Warsi said Mr Johnson was hoping to attract support from right-wing Conservatives for an eventual leadership bid, and called for an independent inquiry into Islamophobia in the party.

Mr Johnson received support from some quarters, with Conservative MP Nadine Dorries saying he "did not go far enough".

Last week, Denmark enforced a ban on the Islamic full-face veil in public spaces, with anyone wearing a garment that hides the face in public risking a fine.

Human rights campaigners have slammed the ban as a violation of women's rights, while supporters argue it enables better integration of Muslim immigrants into Danish society. Wearing a burqa, which covers the entire face, or the niqab, which shows only the eyes, in public will lead to a fine of 1,000 kroner (S$212).

Limitations on wearing face veils in public have already been enacted in France, Belgium, the Netherlands, Bulgaria and Austria.

BLOOMBERG, AGENCE FRANCE-PRESSE