More than half of voters believe Boris Johnson should not face disciplinary action for his comments about the burka, according to a new poll.

The ComRes survey for the Sunday Express found 53% opposed to punishment for the former foreign secretary, against 40% who said he deserved to be disciplined.

The poll was released as Mr Johnson returned to the UK from a holiday in Italy amid an escalating civil war within the Conservative Party over his description of Muslim women in face-covering veils as looking like letter-boxes or bank robbers.

Brexit-backing MP Jacob Rees-Mogg denounced the investigation launched into Mr Johnson’s remarks as a “show trial” motivated by Mrs May’s personal rivalry with a man many see as her likely successor.

The Sunday Times reported that four Cabinet ministers had privately expressed dismay at the handling of the case.

And Tory MP Andrew Bridgen, a supporter of Mr Johnson, warned of “open warfare” in the Conservative Party if he was suspended in such a way that he could not take part in a future leadership contest.

MP portraitsAndrew Bridgen warned of ‘open warfare’ if Boris Johnson is suspended from the Tory Party (Chris McAndrew / UK Parliament)

Mr Johnson made no comment to waiting reporters as he arrived back at his Oxfordshire home on Saturday evening, but is expected to break his silence in his regular Monday column in the Daily Telegraph, where his controversial comments were first printed six days ago.

Far-right US activist Steve Bannon, who was in contact with Mr Johnson during his recent visit to the UK, urged him not to “bow at the altar of political correctness” by apologising.

The former aide to Donald Trump told the Sunday Times that Mr Johnson had “nothing to apologise for.”

Amid complaints from supporters of an attempt to gag Mr Johnson, the ComRes poll found that 60% of respondents believe that rights to free speech are being weakened, against just 5% who said they were strengthening.

Support for Mr Johnson was markedly higher among older generations, with 77% of over-65s and 63% of 55-64 year-olds saying he should not face discipline, while 62% of 18-24 year-olds and 55% of those in the 25-34 age-group saying he should.

The poll found that Theresa May remains voters’ preferred leader of the Conservatives, by a margin of 26% to 24% over Boris Johnson, with 42% opting for “neither”.

Meanwhile, there were signs of concern among Tory donors over Mrs May’s handling of the issue.

David Wall, the secretary of the Midlands Industrial Council, whose members give millions to the Tories each year, described the row as “an argument over relatively nothing”.

Mr Wall told the Sunday Telegraph: “I don’t think his situation needs to be referred to a disciplinary committee of the party.

“I don’t think he expressed himself in an offensive way at all. I have talked to several colleagues and they have the same view as I do. What on earth is all the fuss about?”

And City financier Jeremy Hosking, who has given £375,000 since 2015, told the paper: “The cynical and opportunistic response of some leading Conservatives in condemning him seems so transparent in its motivation as to be laughable.”

Mr Johnson was subjected on Saturday to a brutal attack by a former close aide to David Cameron, Lord (Andrew) Cooper, who accused him of “casual racism” and “courting of fascism”.

“He will advocate literally anything to play to the crowd of the moment,” said the Tory peer. “His career is a saga of moral emptiness and lies; pathetic, weak and needy; the opposite of strong.”

But Mr Bridgen told the Sunday Express: “If Boris is suspended it will be open warfare in the Conservative Party.

“If Theresa May dares engineer a leadership contest while Boris is suspended it will be World War Three.”