Boris Johnson is to face investigation by an independent panel following complaints that his comments on the burka breached the Conservative Party's code of conduct.
It is understood that the process is being triggered automatically after the receipt of a number of complaints over the former foreign secretary's comparison of Muslim women in face-covering veils with bank robbers or letter boxes.
The party declined to comment on the disciplinary procedure.
A spokesman said only: "The code of conduct process is strictly confidential."
But party rules state that formal complaints against Conservative representatives will be investigated "in a timely and confidential manner" by someone "with appropriate experience and no prior involvement in the complaint".
"The investigation should be thorough, impartial and objective, and carried out with sensitivity and due respect for the rights of all parties concerned," says the code of conduct.
News of the investigation emerged as Mr Johnson was reprimanded for breaching rules for former ministers by failing to seek advice before taking up a job as a newspaper columnist after leaving the Cabinet.
The Advisory Committee on Business Appointments (Acoba) said it was "unacceptable" that he signed a contract with the Daily Telegraph on July 12 - three days after quitting as foreign secretary - but did not ask for its advice on the propriety of the move until 14 days later, by which time his appointment was public knowledge.
This was "a failure to comply with your duty" under the Government's business appointment rules for former ministers, Acoba told Mr Johnson.
It was in his weekly Telegraph column that Mr Johnson made comments which sparked demands for an apology from a host of figures from all sides of politics, including Theresa May, Tory chairman Brandon Lewis and Scottish Conservative leader Ruth Davidson.
The founder of the Conservative Muslim Forum, Lord Sheikh, was one of those to write to Mr Lewis to demand "serious action" in response to what critics described as Islamophobia by the former Cabinet minister.
Mr Johnson, who is holidaying abroad, has made no response to demands for an apology.
And there was no immediate comment on the news that he faces investigation.
But sources close to the Uxbridge MP made clear earlier this week that he stands by the article on Monday, in which he argued against a burka ban, but said that the garment was "oppressive" and "ridiculous".
Disciplinary action could lead to Mr Johnson being suspended or even expelled from the Tories, 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.
Under party rules, the head of the investigation may dismiss complaints that are obviously trivial or lacking in merit and those which cannot fairly be investigated.
Mr Johnson will be given the opportunity to make a statement providing "any evidence or details that will help to establish their position".
A panel of at least three people, appointed by Mr Lewis, will include one independent member, one representative of the voluntary party and one nominated by the chair of the backbench 1922 Committee.
They will decide whether there is a potential breach of the code, which requires all party representatives to "encourage and foster respect and tolerance" and not to "use their position to bully, abuse, victimise, harass or unlawfully discriminate against others".
The panel reports its findings to the party chairman but the final decision on any sanction could go to Mrs May as leader or to the party's board.
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.
The row has highlighted deep divisions within the party, with former attorney general Dominic Grieve saying he would quit the Tories if Mr Johnson became leader.
Prominent backbench eurosceptic Jacob Rees-Mogg said Mrs May was wrong to tell 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 apologise. 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."
And 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".