Pressure is mounting on Boris Johnson to back down over his controversial comments about women in burkas as senior Conservative figures lined up to call for an apology.

Culture Secretary Jeremy Wright and Ruth Davidson, the leader of the Scottish Tories, were among those who said his remarks in his Daily Telegraph column had crossed a line.

But supporters of the former foreign secretary claimed he was being targeted in an attempt to ward off a possible leadership challenge in the autumn.

Mr Johnson, who is on holiday abroad, has not been seen in public since his article – in which he said women wearing the Muslim face veil looked like letterboxes and bank robbers.

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Nadine Dorries has said a campaign is under way to prevent Boris Johnson becoming leader (PA)

However, sources close to him have made clear that he stood by the article in which he argued against a burka ban of the kind adopted by some European countries.

Some in the party saw the stand-off as a continuation of the ongoing struggle over Brexit following Mr Johnson’s resignation last month over Theresa May’s Chequers plan.

Tory backbencher Nadine Dorries said the attacks were being led by those on the Remain wing who feared a possible leadership challenge.

“The campaign to stop Boris becoming leader is underway in a very crass and cack handed way,” she wrote on Twitter.

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Lord Pickles has suggested Boris Johnson could face disciplinary action (Victoria Jones/PA)

However, the former cabinet minister and Conservative party chairman Lord Pickles warned that Mr Johnson could face disciplinary action unless he was prepared to back down.

He said the party has “various procedures” if an official complaint were to be made, although he said it would be “a very big leap” to suggest that could lead to his expulsion.

“Pretty much inconceivable, but you never know how these things develop,” he told the BBC Radio 4 Today programme.

Meanwhile, the founder and president of the Conservative Muslim Forum, Lord Sheikh, stepped up his demands for the Conservative whip to be withdrawn.

He said he had written to party chairman Brandon Lewis calling for “severe action” against Mr Johnson for his “inflammatory” remarks.

“I think to a certain extent they’re racist. In a way it is racist. These words are very inflammatory,” he told Sky News.

“I believe they will cause problems with race relations. I believe it will encourage bigotry in this country.”

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Ruth Davidson said Mr Johnson’s comments were gratuitously offensive (Jane Barlow/PA)

After Mrs May backed calls on Tuesday for Mr Johnson to apologise, Mr Wright said that while important issues such as the wearing of burka were discussed publicly, politicians needed to choose their language with care.

“When you are discussing a subject such as this, then I think describing it as people looking like letterboxes isn’t helpful,” he told the Today programme.

Ms Davidson, however, went further, saying Mr Johnson’s comments had been “gratuitously offensive” and that he needed to apologise.

“I think that this wasn’t an off-the-cuff slip, he wrote a column, he knew exactly what he was doing and I think it crossed from being provocative and starting a debate and actually it became rude and gratuitous,” she said.

Former attorney general Dominic Grieve, who has in past clashed with Mr Johnson over Brexit, said his comments were “very embarrassing” and warned he would leave the party if he ever became leader.

“I don’t regard him as a fit and proper person to lead a political party and certainly not the Conservative Party,” he told the BBC Radio 4’s The World at One.

“He is somebody who seems to me to pursue an agenda that is entirely self-referential.”

Former minister Anna Soubry predicted “many” members of the party’s moderate One Nation wing would quit if Mr Johnson took over.

“Dominic Grieve is right,” said Ms Soubry. “I believe many One Nation Tories would not stay in the Conservative Party should Boris Johnson become leader.”