Pressure is mounting on the Conservative Party leadership to decide whether to take action against Boris Johnson for his controversial comments about Muslim woman wearing the burka.

Two days after party chairman Brandon Lewis ordered the former foreign secretary to apologise, Mr Johnson was maintaining his silence on a holiday break as supporters and critics exchanged furious brickbats.

The founder of the Conservative Muslim Forum, Lord Sheikh, has written to Mr Lewis demanding “serious action”, while the call for an apology has been backed by Cabinet ministers including Jeremy Wright and Penny Mordaunt as well as Prime Minister Theresa May herself.

Disciplinary action could lead to Mr Johnson being suspended or even expelled if he was found to be in breach of the Tory code of conduct, but would risk igniting civil war in a party many of whose members see him as the best option to succeed Mrs May as leader.

Ruth Davidson Scottish Conservative leader Ruth Davidson said Boris Johnson’s comments on the burka were ‘gratuitously offensive’ (Yui Mok/PA)

Scottish Conservative leader Ruth Davidson backed calls for an apology, saying it was “gratuitously offensive” for Mr Johnson to use an article in the Daily Telegraph to describe women in face-covering veils as looking like letterboxes or bank robbers.

“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,” said Ms Davidson.

Meanwhile, former attorney general Dominic Grieve said he would quit the party if Mr Johnson became leader, while ex-minister Anna Soubry said that “many” One Nation Tories would also leave.

Critics have accused Mr Johnson of using the row to win right-wing support in any future leadership battle. A Sky News poll found that 45% of voters thought he should apologise, while 48% thought he should not.

But supporters claim that the Brexiteer is being targeted in an attempt to ward off a possible challenge to Mrs May in the autumn, when negotiations with the EU will come to a head.

Prominent backbench eurosceptic Jacob Rees-Mogg said there was no need for Mr Johnson to apologise.

Asked about Mrs May’s handling of the controversy, Mr Rees-Mogg told LBC radio: “She’s clearly wrong to have asked him to have apologised. It’s not the job of the Prime Minister to tell backbenchers what to do.

“We are not bound by collective responsibility or that type of discipline and it is the wrong approach to party discipline.”

An imam who has previously criticised the burka said Mr Johnson should not “apologise for telling the truth”.

In a letter to The Times, Taj Hargey of the Oxford Islamic Congregation said there was “no Koranic legitimacy” for the burka, which he said had been used as part of a “gateway theology for religious extremism and militant Islam”.

Shortly after the publication of his article on Monday, a source close to Mr Johnson said that it was “ridiculous” that his views should be under attack.

“We must not fall into the trap of shutting down the debate on difficult subjects,” the source said.

“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.”