Letter-boxes!

British Prime Minister Theresa May on Tuesday backed calls for her former foreign minister Boris Johnson to apologise for disparaging comments he made about Muslim women wearing burqas -- but he branded his critics "ridiculous".

May said his remarks "have clearly caused offence" and agreed with the chairman of her Conservative party, Brandon Lewis, who had asked Johnson to apologise.

"I do think that we all have to be very careful about the language and terms we use. And some of the terms Boris used describing people's appearance obviously have offended," the prime minister said.

In a column in Monday's Daily Telegraph newspaper, Johnson said women wearing the full face veil looked like "bank robbers" or "letter-boxes", prompting accusations of Islamophobia.

But the former top diplomat, who has a reputation for causing controversy and quit May's cabinet last month in protest at her Brexit plan, 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 Johnson told reporters.

In his article, Johnson said he opposed a ban on face-covering veils, but added that it was "absolutely ridiculous that people should choose to go around looking like letter-boxes".