- Video report by ITV News Political Correspondent Paul Brand
Conservative backbenchers are said to be "in despair" over the appointment of Chief Whip Gavin Williamson as the new Defence Secretary.
His promotion follows the shock resignation of Sir Michael Fallon on Wednesday in the wake of sexual harassment claims at Westminster.
An unnamed Tory former minister told the Press Association that some of the party's MPs were in "head-in-hands despair" at Mr Williamson's "bizarre" appointment.
"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," the MP said.
"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."
And in an apparent reference to Mr Williamson's promotion, Tory MP Sarah Wollaston tweeted: "There are times when offered a job that it would be better to advise that another would be more experienced and suited to the role."
Her colleague Anna Soubry replied: "It does rather look like he picked himself a plum job."
Ms Wollaston later told BBC Radio Four's World at One that Mr Williamson was an "extremely likeable and respected chief whip" but said that part of the role was to advise the PM on the "most experienced and suitable person" to take on the job.
Sir Michael stepped down after it emerged he repeatedly put his hand on a female journalist's knee at a dinner in 2002, prompting a cabinet re-shuffle.
He apologised for his behaviour saying it had "fallen below the standards required" for his role.
Following his promotion, Mr Williamson said in a statement he was "determined to ensure that the armed forces receive the recognition they deserve for the great work they do, including through the Armed Forces Covenant, and that they evolve both to meet the changing threats that we face and to ensure that they properly represent the modern society that they defend".
A spokesman for the Prime Minister said Mr Williamson was not involved in discussions about the reshuffle following Sir Michael's resignation, as chief whips often are.
The spokesman added: "Gavin Williamson was an excellent and hard-working chief whip and the Prime Minister thinks he will make an excellent Defence Secretary.
"The PM is confident in the operation of the whips' office during her premiership."
Sir Michael's resignation has prompted speculation others may follow suit after it emerged there was a list of MPs accused of inappropriate behaviour circulating.
The unverified list is said to contain the names of up to 40 MPs.
When allegations of sexual misconduct first came to light last week, ministers were warned that "serious action" would be taken by the Prime Minister where necessary.
International Trade Minister Mark Garnier is being investigated over claims he asked his secretary to buy him sex toys.
Ex-minister Stephen Crabb is also facing accusations he sent explicit texts to a 19-year-old woman he interviewed for a job.
Claims First Secretary Damian Green touched a young activist's knee are also being looked into.
Transport Secretary Chris Grayling said he regretted Sir Michael's decision to resign but denied his exit was the Government's attempt to "clean out the stables" - a phrase used by Scottish Tory leader Ruth Davidson.
"I am very sorry to see Sir Michael Fallon step down from government, he's done a very good job as Defence Secretary, I think it is a great shame that he's been in the position where he has had to leave in these circumstances," he said.
"I know Gavin Williamson will do a very good job in his place and the important thing is we get on with governing the country, taking us through the Brexit process and delivering our domestic agenda.
"Yes, it's been a difficult week at Westminster but the government's job is to get on with the job."
- Who is Gavin Williamson?
Gavin Williamson got his big break when David Cameron made him his Parliamentary Private Secretary from 2013 to 2016.
When Cameron resigned, Williamson backed Theresa May - who rewarded him by promoting him to her Cabinet as Chief Whip.
His promotion to Defence Secretary is the first time the 41-year-old has held ministerial office.
Born in Scarborough, North Yorkshire to Labour-supporting parents, Williamson did not enter Parliament until 2010 when he became MP for South Staffordshire.
He is known in Westminster for keeping a pet tarantula named Cronus in a glass box on his desk, which is said to have provided added menace when dealing with errant MPs in his role as Mrs May's 'enforcer'.
"You have to look at all different ways to persuade people to vote with the Government and it's great to have Cronus as part of the team," he said on his appointment as chief whip, with responsibility for maintaining discipline among Tory MPs.
Married with two daughters, his background is in manufacturing and design at a pottery in Staffordshire and an architectural design company.
He was awarded a CBE in Mr Cameron's resignation honours for political and public service.
- Julian Smith
Julian Smith has been announced as Mr Williamson's replacement as Chief Whip.
- Esther McVey
Tatton MP Esther McVey, who served as a work and pensions minister from 2012-15, has been appointed Deputy Chief Whip.