A Tory backlash has erupted over the appointment of Gavin Williamson as the new Defence Secretary with Theresa May accused of appearing “pathetically weak” amid claims she has rewarded loyalty rather than ability and experience.

The appointment of the 41-year-old Chief Whip, who was the Prime Minister’s campaign manager during the 2016 Conservative leadership race, came as a surprise at Westminster.

Mr Williamson, the MP for South Staffordshire, entered the UK Parliament in 2010 and has, until now, never held a ministerial position. Before entering the whips’ office, he was parliamentary private secretary to David Cameron.

Asked why the Prime Minister had appointed her little-known colleague, who had no experience at the Ministry of Defence, Mrs May’s spokesman replied: “Gavin Williamson was an excellent and hard-working Chief Whip and the Prime Minister thinks he will make an excellent Defence Secretary.”

Given there have been claims whips had used MPs’ indiscretions in the past to give the party leverage over them on, say, votes, the spokesman was asked if there had been any concerns raised with Mrs May in this regard. He replied: “The PM is confident in the operation of the whips' office during her premiership."

The spokesman made clear Mr Williamson was not involved in discussions about the reshuffle following the resignation of Sir Michael Fallon as chief whips often are.

The new Secretary of State admitted to being “absolutely flabberghasted” by his appointment.

"Few people can have such a great honour than to be responsible for overseeing the Armed Services of this great country and helping to formulate policy in terms of how we move forward.

"Walking through the doors of the MoD I immediately felt a real sense of responsibility on my shoulders. I am under no illusions that this is a massive challenge and a huge responsibility. It is one that I hope I will be able to meet,” declared Mr Williamson.

But several colleagues expressed astonishment by his appointment given the breadth and depth of defence experience elsewhere in the party and across government.

One Tory MP suggested the move made the PM look "pathetically weak".

He said: “It looks like Williamson has played her. When you have a wide range of experienced alternate options for Defence Secretary, why would you do that?”

One ex-Conservative minister suggested some of his parliamentary colleagues were in “head in hands despair”.

He explained: "The feeling is it's just a move that demonstrates Theresa May's own weakness by allowing the guy, who suggested to her that Fallon should go, to take that job.

"It's a bizarre appointment from somebody who's so shell-shocked she doesn't know which direction to turn in and so is listening to the person she just likes and trusts rather than having a view about it herself,” added the backbencher.

One serving minister was even blunter, claiming the appointment proved Mrs May had “gone mad”.

A female Tory colleague, asked if his appointment had gone down badly, replied: “You bet your f***king life.”

Sir Michael quit on Wednesday evening after admitting his behaviour had "fallen below the high standards required" in the role and acknowledging that what might have been acceptable in the past was no longer appropriate.

The 65-year-old Scot was one of the most senior ministers to be caught up in a wave of allegations of improper behaviour swirling around Westminster, after admitting putting his hand on the knee of radio presenter Julia Hartley-Brewer 15 years ago. Speculation is rife that more claims about Sir Michael were about to be made and it was the “PM’s idea” that he should go.

Mr Williamson was replaced as Chief Whip by his former deputy Julian Smith, born in Stirling, while Tatton MP Esther McVey - a former TV presenter who served as a minister in Mr Cameron's government - was made Deputy Chief Whip.