THERESA May has repeatedly failed to condemn Boris Johnson’s language as Islamophobic after he said Muslim women wearing burkas looked like letter boxes and bank robbers.

The Prime Minister insisted Mr Johnson's comments had clearly caused offence and called on him to say sorry, but refused to say whether his remarks were prejudiced.

It came as First Minister Nicola Sturgeon accused Mr Johnson of deliberately indulging in “dog whistle politics”, branding his words “reprehensible and disgraceful”.

Mrs May spoke out after sources close to the former foreign secretary indicated he would not retract his comments.

She said she agreed with Conservative chairman Brandon Lewis, who called on Mr Johnson to apologise.

She added: “I am very clear about our position on the issue of the burka, which is that women should be able to choose how to dress.

“It’s up to a woman to decide how to dress; it’s not up to other people to tell a woman how to dress.

“But obviously these issues are ones that are openly discussed, and it’s right that we have discussion about issues like this.

“But, in doing that, we all have to be very careful about the language and terms that we use.

“Some of the terms that Boris used in describing people’s appearance obviously have offended people, and so I agree with Brandon Lewis.”

When The Herald repeatedly pressed Mrs May on whether she considered Mr Johnson’s language Islamophobic, she failed to address the question and glanced nervously at her official spokesman.

She later said Mr Johnson was “wrong to use that language”.

Earlier, Ms Sturgeon accused Mr Johnson of being “deliberately provocative”, adding: “I’m a wee bit cynical about these calls for him to apologise. Boris Johnson didn’t make a mistake – he knew what he was doing.

“He deliberately made those comments, and it’s a kind of dog whistle politics that he’s indulging in.

“It is Islamophobia, and I think it’s pretty outrageous. Hopefully if she hasn’t already, the Prime Minister will be unequivocal in condemning it.”

Mr Johnson made the remarks in his Daily Telegraph column in response to Denmark's introduction of a ban on burkas in public places.

He argued against banning the face-covering veils, but wrote: "It is absolutely ridiculous that people should choose to go around looking like letter boxes.”

He also insisted that "if a female student turned up at school or at a university lecture looking like a bank robber" then those in authority should feel entitled to ask her to remove her veil.

The article sparked an angry backlash from Muslim organisations and MPs, who accused him of stoking Islamophobia for political gain.

Former Tory chair Baroness Warsi labelled the comments “bigotry”, and repeated her call for an independent inquiry into Islamophobia in the Conservative Party.

The Muslim Council of Britain said Mr Johnson's "intentional usage of the words" appeared to be an attempt to "pander to the far right".

It said any inquiry into Islamophobia must look into Mr Johnson's previous "incendiary remarks" that "Islam is the problem" and that "Islamophobia was a natural reaction".

A source close to Mr Johnson, who is said to be considering a leadership bid, made clear he would not retract his comments.

They 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.

"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."