The prime minister said the former foreign secretary’s comments had „clearly caused offence”, as controversy around his words grew on Tuesday.
Her intervention followed a wave of anger from senior Conservatives, who slammed Mr Johnson’s remarks as Islamophobic and accused him of partaking in dog-whistle politics to stoke a future populist leadership bid.
The chair of the Conservative Muslim Forum also told The Independent his words „pander to the far-right narrative that Muslims do not belong in this country”.
But Mr Johnson has refused to back down, with friends claiming it is „ridiculous” to criticise his comments and accusing party chiefs of „shutting down the debate”.
Taking time out of an official trip to Scotland in a bid to try and get a grip on the controversy, Ms 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 [Tory chairman] Brandon Lewis [that Mr Johnson should apologise].
„What’s important is do we believe people should have the right to practise their religion and, in the case of women and the burqa and niqab, to choose how they dress. I believe women should be able to choose how they dress.”
The prime minister said she believed it was right to discuss such issues openly, but reiterated the need for care in the language used and added that Mr Johnson’s words, published in an article for The Daily Telegraph, had „clearly caused offence”.
The furore was sparked when Mr Johnson wrote in his column that Muslim women wearing headdresses were choosing „to go around looking like letter boxes”.
He then added that if a women turned up to his surgery wearing a face covering, he should „feel entitled” to ask her to remove it.
The former London mayor went on: „If a female student turned up at school or at a university lecture looking like a bank robber, then ditto.”
The backlash against Mr Johnson began when his former deputy at the Foreign Office, minister Alistair Burt, said he would never have made such comments and argued that there was „a degree of offence” in them.