The promotion of Theresa May’s chief whip Gavin Williamson to the prized role of defence secretary hasn’t gone down well with some of her MPs, party insiders told BuzzFeed News.

The shock resignation of Sir Michael Fallon after allegations of improper behaviour opened up a big role in the cabinet, and Williamson wasn’t one of the names floated as a potential replacement.

Some insiders were stunned when Williamson’s appointment was announced on Thursday morning. A minister told BuzzFeed News it had "gone down like a cup of cold sick". “Person with power over promotions promotes himself shock!” one senior Tory MP said.

Another senior MP said backbenchers are “livid” at Williamson’s elevation, and that some will now feel even less loyalty to a leader whose grip on office was already tenuous.

“It’s her biggest and probably last mistake,” the MP told BuzzFeed News.

The appointment only underlines the weakness of 10 Downing Street, the MP added, and “demonstrated that what counts is dark arts not good politics”.

The feeling among backbench colleagues is that Williamson and his deputy, Julian Smith, who has been promoted to chief whip to replace Williamson, have exploited May’s weakness to advance their own careers.

Sarah Wollaston MP, chair of the health select committee, tweeted: “There are times when offered a job that it would be better to advise that another would be more experienced & suited to the role.”