Boris Johnson’s longest-serving aide resigned after a power struggle between rival factions in government.
Internal tensions spilled into the public domain on Wednesday night after director of communications Lee Cain announced he would be stepping down next month after a row over whether he should be made chief of staff.
Mr Cain’s resignation came after a battle between opposing factions at the heart of government.
Carrie Symonds, Mr Johnson’s fiancee, was reportedly against Mr Cain's promotion to chief of staff.
She was backed by Allegra Stratton, winner of the recent contest to be new Downing Street spokeswoman.
On the opposing side was Dominic Cummings, Mr Johnson’s influential chief aide, who was pushing for Mr Cain’s promotion.
Mr Cummings and Mr Cain emerged as the losers in the power struggle.
Mr Cummings reportedly threatened to quit over the news of Mr Cain’s resignation.
Mr Cain worked alongside Mr Johnson on the Vote Leave campaign as well as advising him while he was foreign secretary.
The power struggle came on the same day the UK hit the grim milestone of 50,000 coronavirus deaths – becoming the fifth country in the world to hit the figure.
Domicins Cummings sparks Tory internecine warfare
Conservative MPs expressed despair about the political infighting as the country struggled to contain the second coronavirus wave.
Sir Charles Walker, vice-chairman of the powerful 1922 Committee representing backbenchers, said Mr Johnson was being undermined by his advisers and that MPs had been "unhappy about the No 10 operation for some time".
He told BBC Radio 4's Today programme: "We get very upset when his equilibrium is undermined by people supposed to be working for him.
“We’ve heard far too much from advisers over the last 18 months.”
Mr Walker said MPs felt excluded from the decision-making process under Mr Johnson’s current team.
Asked whether Mr Cummings should resign, he said: “Again, this is a matter for the PM whether Dominic stays.”
Housing Secretary Robert Jenrick dismissed suggestions the government was distracted by the infighting.
"The government is getting on with all the things that matter to the public," he told the BBC.
But Labour Party leader Sir Keir Starmer said the situation was pathetic.
He told LBC Radio: "I think millions of people will be waking up this morning, scratching their heads, saying what on earth is going on?
"We're in the middle of a pandemic, we're all worried about our health and our families, we're all worried about our jobs, and this lot are squabbling behind the door of No 10. Pull yourselves together, focus on the job in hand."
In his resignation statement, Mr Cain confirmed that he was offered the chief of staff position.
But the Brexiteer said he decided to leave regardless of the offer.
He said: "It has been a privilege to work as an adviser for Mr Johnson for the last three years – being part of a team that helped him win the Tory leadership contest, secure the largest Conservative majority for three decades – and it was an honour to be asked to serve as the prime minister's chief of staff.
"I would like to thank all the team at No 10, including the many unsung and incredibly talented civil servants, for their hard work and support during the last 18 months.
"And most of all, I would like to thank the prime minister for his loyalty and leadership.
"I have no doubt that under his premiership the country will deliver on the promises made in the 2019 election campaign and build back better from the coronavirus pandemic."
Mr Johnson thanked his close ally for his “extraordinary service”.
He said: "He has been a true ally and friend and I am very glad that he will remain director of communications until the new year and to help restructure the operation.
“He will be much missed."
