Theresa May is facing a furious backlash from grassroots Conservative members and donors over the party's "malevolent" and "discourteous" decision to investigate Boris Johnson over his burka comments.

One local association president said Mrs May was “at best an embarrassment, and at worst a humiliation for our great party and country”, while major donors said it was an “argument over relatively nothing”.

Number 10 was forced to deny reports of a rift with party chairman Brandon Lewis, after he said on Tuesday Mr Johnson should apologise for saying women wearing burkas looked like robbers and letter boxes.

The Prime Minister was given no notice ahead of Mr Lewis's criticism of Mr Johnson, forcing Mrs May to back Mr Lewis during a pre-planned clip for television cameras on a visit to Scotland a few hours later.

The party insists it has "a duty" to investigate Mr Johnson's remarks after complaints were received about them, however the investigation has not yet reached the stage when a formal panel is appointed.