THERESA May can expect “open warfare” if she suspends Boris Johnson over his burka comments, a close ally of the former Foreign Secretary has claimed.

Johnson’s remarks in a newspaper in which he compared women wearing the niqab to letter boxes and bank robbers, have split the party with a slew of Cabinet ministers attacking the Prime Minister and the leadership over the weekend.

Party chair Brandon Lewis has instructed an independent panel to investigate Johnson following dozens of complaints alleging the comments breached the Tories’ code of conduct.

Jacob Rees-Mogg, leader of the party’s anti-EU European Research Group said May wanted a “show trial” for Johnson as she felt threatened by him.

Tory MP Andrew Bridgen told a Sunday newspaper there would be “open warfare” in the party if Johnson was suspended.

“If Theresa May dares engineer a leadership contest while Boris is suspended it will be world war three,” he said.

The former Cabinet minister refused to comment as he arrived back in England from a holiday in Italy on Saturday night.

There was praise for the former Foreign Secretary from Donald Trump’s outspoken strategist and former adviser Steve Bannon.

Bannon, the figurehead of the alt-right movement praised Johnson as someone who would make a truly “a great prime minister”.

He told reporters: “Boris just needs to be Boris – true to his nature and his calling – and I think he has potential to be a great prime minister, not a good one.”

Bannon has been in contact with Johnson since he resigned from government last month, although he added: “Any conversations I have with active political figures are confidential – I consider Boris Johnson someone who understands the physics in the ebb and flow of events. Those individuals are rare.”

The American right-winger also had kind words for Tommy Robinson, the leader of the English Defence League, jailed last month for contempt of court.

On Robinson, Bannon said: “Tommy is not just a guy but a movement in and of himself now.”

Robinson was released on bail this month after an appeal court ruled that he should face a retrial on technical grounds.

The endorsement from Bannon came as Damian Green, May’s former de facto deputy, said he feared Johnson was “being turned into a martyr by the alt-right”.

Writing in a newspaper, Green said: “I am particularly concerned by reports that President Trump’s sacked adviser Steve Bannon is forming a Europe-wide far-right campaign group – and has been in touch with Boris.

“I hope that no Conservative politician, including Boris, is taking advice from him about how the Conservative party should behave.”

Tory peer Lord Cooper accused Johnson of “casual racism” and courting fascism.

In a tweet Cooper said: “The rottenness of Boris Johnson goes deeper even than his casual racism and his equally casual courting of fascism. He will advocate literally anything to play to the crowd of the moment.

“His career is a saga of moral emptiness and lies; pathetic, weak and needy; the opposite of strong.”