A row has broken out at the heart of Downing Street as one of Boris Johnson’s most senior aides – and a close ally of Dominic Cummings - resigned amid bitter infighting. Lee Cain announced he would step down as director of communications on Wednesday night after ministers and advisers including Johnson’s fiancee, Carrie Symonds, are said to have protested over his planned promotion to chief of staff. Cain’s resignation also threw into doubt the future of Cummings, Johnson’s most senior and high-profile adviser, with one insider telling the Guardian that he could depart No 10 alongside Cain. Cummings was said to be furious that Cain, a fellow Vote Leave campaigner who was partly credited with bringing him into No 10, had in effect been forced out. There was also speculation about other potential departures from Johnson’s inner circle. Meanwhile some Tory MPs were buoyed by the turmoil, with one claiming it was a chance for Johnson to get “out of the grip of these people”. The row appears to reflect internal turmoil and dysfunction in No 10 at a time when the UK surpassed 50,000 deaths from the coronavirus, according to official figures. Critics of the government have repeatedly cited how tensions between senior members of Johnson’s team – and his refusal to sack Cummings earlier this year – have hindered the country’s efforts to get on top of the pandemic. As the internal rows were thrown into the public spotlight on Wednesday, the extent of division and disagreement within Johnson’s senior team emerged. About a week ago Johnson offered the new chief of staff position to Cain, 39, who was regarded as one of his most loyal and low-key longstanding staffers, and the adviser had been considering whether he should accept. Friends indicated that Cain had been thinking about leaving Downing Street “once the government had got over the hump of Covid” – partly because of the relentless nature of the job and also because of clashes with the incoming Downing Street spokeswoman Allegra Stratton – until Johnson made his unexpected offer. Cain is known to have been uneasy with the choice of Stratton, also 39, for press secretary and he had previously clashed with Symonds. His pugnacious style with MPs and special advisers meant a number expressed outrage to No 10 at his potential promotion, which would have made him one of three key people in the prime minister’s inner circle, along with Cummings and cabinet secretary Simon Case. Allies of Cain said he had been acting as an effective chief of staff for some time, taking part in major strategic decisions including a Chequers summit in April about easing lockdown. Special advisers’ WhatsApp groups were “nuclear” on Wednesday night, one Whitehall source said, with a number speculating about what Cain’s departure could mean for the planned communications shake-up across Westminster. A number of Conservative MPs expressed delight at the row. One MP said it was “a chance to get the old Boris back and out of the grip of these people”. Another said MPs would be “over the moon” if Cummings also departed. One senior MP said they believed it was a sign that the battle for influence with the prime minister was being slowly won by Symonds and her allies, rather than Cummings. “Boris must take this chance to pivot back to being the liberal unifier, sack a chunk of these cabinet no-hopers and start a new team, but it does look like tonight that Carrie and those urging a more inclusive Boris are winning.” Cain will leave No 10 at the end of the year, saying he had made the decision “after careful consideration”. It is understood he met Johnson in Downing Street on Wednesday evening but ultimately decided to go. “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,” he said. “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.” James Slack, the prime minister’s official spokesman and the former political editor of the Daily Mail, is set to take over as director of communications. Allies of Cain said he had taken time off over the past few days to consider his position, described by one friend as a “circuit breaker”. The comment was meant jokingly but was also a nod to the pressures Cain had been under. Cain is a longtime adviser to Johnson, including during his time as foreign secretary and while on the backbenches after his resignation. The pair are particularly close, with Cain staying loyal to Johnson even when the former mayor of London’s star appeared to be fading among Tories. A former Daily Mirror journalist who once dressed as a chicken to taunt Tory prime minister David Cameron, Cain is close to Cummings from their time working on the Vote Leave campaign, and is partly credited with bringing him into Downing Street. A new chief of staff is set to take over the day-to-day running of the Downing Street operation, currently overseen by Cummings, to allow him to step back and focus on key policy objectives, such as Whitehall reform. Cummings’ journey to Durham from London at the height of lockdown in the first wave of the coronavirus pandemic – and Johnson’s refusal to sack him as a result – became one of the biggest government crises of the year. It was revealed following an investigation by the Guardian and Daily Mirror. Critics said the prime minister was so dependent on Cummings, who has vowed to overhaul the civil service and hire “weirdos and misfits” to join the No 10 team, that he risked the wrath of the public and Tory MPs rather than lose his most senior aide. One former Whitehall colleague of Cain’s said: “The backbenches will be heartened to see a broader group of people influencing the prime minister and Number 10. Lee Cain was a divisive figure although I don’t think anyone will cheer that there is this upheaval in the middle of a pandemic. James Slack is well liked and, with Allegra Stratton’s appointment too, people will be reassured by this change in direction.” Johnson paid tribute to Cain and said he would be much missed. “‘I want to thank Lee for his extraordinary service to the government over the last four years,” 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.” A Labour spokesperson said: “On the day the UK became the first country in Europe to report 50,000 coronavirus deaths and the public endure another lockdown, Boris Johnson’s government is fighting like rats in a sack over who gets what job. “It is precisely this lack of focus and rank incompetence that has held Britain back. The public deserve better than this incompetent and divided Conservative government.”