Meanwhile Mr Halfon, the education select committee chair, said that while he was a “naturally loyal” Conservative, there had been a “loss of integrity” as well as a “failure of policy”.
Meanwhile, several Tory MPs have started publishing the letters they sent earlier this year, including Jo Gideon and Anthony Browne, a long-term ally of Mr Johnson who worked with him at City Hall.
All eyes on 1922 committee
Sir Graham Brady, the chair of the 1922 committee of backbenchers, is expected to come under pressure to make the strength of feeling in the party known to the Prime Minister.
Under current rules, Mr Johnson cannot face a challenge to his leadership until next June, when it will be 12 months since the most recent vote of confidence.
But backbenchers have launched a plot to unseat him sooner than this by getting rebels elected to the 1922 committee’s executive and then changing the rules to allow an earlier vote and possibly with a lower threshold.
At the moment, the chair of the committee must receive 54 letters from MPs – representing 15 per cent of the parliamentary party – expressing their lack of confidence in the Prime Minister to trigger a vote, and this must be no sooner than a year after the last one.
Mr Johnson won a confidence vote on June 6, with 211 votes in favor of his leadership and 148 against.
How can men in gray suits unseat PM?
One plan under discussion is to add a new rule that would state that if 90 new letters – representing 25 per cent of the party – were submitted, it could trigger a fresh vote immediately rather than having to wait for the full year to elapse.
Alternatively, the threshold could remain the same but the rule about waiting 12 months could be scrapped.
Another scenario is that the current executive of the committee agrees to a rule change to allow an immediate no confidence vote to take place – this could happen as early as Wednesday.
And yet another situation could see Sir Graham leading a delegation of senior backbenchers to visit the Prime Minister and tell him he has lost the support of his party.
Only last month Mr Johnson was asked whether he would listen if the men in gray suits turned up at his door – a reference to the string of senior ministers who visited Margaret Thatcher to tell her to resign after she narrowly survived a leadership challenge by Michael Heseltine . He said at the time: “I love my colleagues and I will always.”
The count for letters calling for the Prime Minister to go is reset after each confidence vote, but rebels are confident they could get the votes they need to oust their leader.
The 1922 committee will meet later on Wednesday and it is expected that nominations will open for the elections, which are due to take place next week.
Sir Graham is expected to remain in his post but the remaining 17 seats of the executive will be contested.
Rebel MPs will back candidates who support a rule change while the Prime Minister’s allies will support those who want to uphold the status quo. The elections themselves will be seen as a proxy vote on Mr Johnson’s leadership.
Once the new executive committee members are installed and if a rule change is brought in swiftly, Mr Johnson could face a new vote of no confidence before summer recess.