Theresa May has backed calls for Boris Johnson to apologise for his comments about Muslim women who wear burkas.

Prime Minister backed Conservative Party chairman Brandon Lewis, who urged Johnson to apologise for saying Muslim women who wear a burka look like “letter boxes” and bank robbers. 

She said his remarks “clearly caused offence”.

Speaking in Scotland, May said: “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.

“So I agree with Brandon Lewis.

“What’s important is do we believe people should have the right to practise their religion and, in the case of women and the burka and niqab, to choose how they dress.

“I believe women should be able to choose how they dress.”

The PM also said: “These are issues that are openly discussed but in having that discussion it’s important that people think carefully about the language that they use, certain terms in describing people’s appearance that Boris Johnson used have clearly caused offence, that’s why I have said I agree with Brandon Lewis.”

Lewis had ordered Johnson to apologise following a hail of criticism over the remarks, which were labelled “bigotry” by former Tory chair Baroness Warsi.

Johnson has so far refused to apologise and dismissed his critics as “ridiculous”.

A source close to the MP said: “It is ridiculous that these views are being attacked – we must not fall into the trap of shutting down the debate on difficult issues,” said the source.

“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.”

Writing in the Daily Telegraph on Monday, Johnson said he did not want Britain to follow European countries like Denmark which have banned the burka and niqab in public places.

But he described the burka as “ridiculous” and “weird” and said women wearing them looked like letter-boxes or bank robbers.

Alistair Burt, who served under Mr Johnson until his resignation as foreign secretary last month, told BBC Radio 4’s Today programme: “I would never have made such a comment, I think there is a degree of offence in that, absolutely right.”

Lady Warsi accused Johnson of adopting the “dog-whistle” tactics of former Donald Trump aide Steve Bannon in the hope of attracting support from right-wing Tories for an eventual leadership bid.

First Minister Nicola Sturgeon said the comments were Islamophobic and “dog whistle politics”.

She said: “My views on Boris Johnson’s comments are that they are reprehensible and disgraceful.

“They are deliberate though, and deliberately provocative.

“I’m a wee bit cynical about these calls for him to apologise. You call for someone to apologise if they have made a mistake, Boris Johnson didn’t make mistake.

“He knew what he was doing and it’s a kind of dog whistle politics that he’s indulging in.

“It is Islamophobia and I think it’s pretty outrageous.”