The stop-Boris campaign may have crossed an important line, according to a well-placed senior Tory MP - in that junior government whips, who are supposed to remain neutral, have been corralling support for Theresa May.

According to my sources, there was a meeting today of Remain MPs organised by one of the whips. At the meeting the MPs were urged to back May, the Home Secretary.

Because whips are traditionally supposed to be completely neutral because of their role in maintaining discipline, a number of the MPs present were uncomfortable.

Complaints have been lodged with Graham Brady, chairman of the 1922 committee of all backbench Tory MPs.

According to sources, junior whips have been rounding up support for Theresa May Credit: Reuters

I am told the chief whip and deputy chief whip have remained strictly neutral in the leadership contest. They are meeting Mr Brady on Wednesday to explain why junior whips are apparently not required to be neutral this time.

The first round of voting for MPs in the leadership contest will be on July 5.

The two most serious candidates are expected to be May and Boris Johnson, with Johnson widely seen as most likely to succeed David Cameron as party leader and prime minister.

Among allies of Cameron there is fury with Johnson for what they see as his defection from their camp to lead the Leave side to further his personal ambition.