Boris Johnson’s No 10 operation has been thrown into turmoil following the dramatic resignation of one of his closest aides.

Lee Cain announced late on Wednesday he was quitting as director of communications amid a bitter internal power struggle in Downing Street.

His departure sparked speculation he could be followed by Dominic Cummings, the Prime Minister’s top adviser seen as the most powerful figure in No 10.

The two men had worked together on the 2016 EU referendum campaign fronted by Mr Johnson and are regarded as close political allies.

But while Mr Cummings was said to be unhappy at the way his friend had been treated, it was reported he had decided to stay on for now to work on the Government’s response to the coronavirus – in particular the acceleration of the mass testing programme.

In his resignation statement, Mr Cain confirmed he had been offered a promotion to the key position of the Prime Minister’s chief of staff.

The move – which would have meant he was one of just a handful of people in No 10 with direct one-to-one access to Mr Johnson – was seen as entrenching the grip of the Vote Leave faction on the Downing Street operation.

However it ran into immediate resistance, with Mr Johnson’s fiancee Carrie Symonds – who has clashed in the past with Mr Cummings – reportedly strongly opposed to the appointment.

Allegra Stratton, the former TV journalist brought in to host televised No 10 news conferences from next year, was also said to have objected to the appointment.

In his resignation statement, Mr Cain – a former tabloid journalist – said he had decided to resign as communications director after “careful consideration”.

He said it had been an “honour” to have been offered the post of chief of staff and he thanked the Prime Minister for his “loyalty and leadership”.

In response, Mr Johnson paid tribute to Mr Cain’s “extraordinary service” to the Government over the past four years.

“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 said.

“He will be much missed.”

It is expected that Mr Cain will be succeeded by James Slack, the Prime Minister’s official spokesman who also held the post under Theresa May.

Unlike Mr Cain who is a political special adviser, Mr Slack, a former Daily Mail journalist, is a member of the permanent Civil Service.

Mr Cain originally joined Mr Johnson when he was made foreign secretary by Mrs May following the 2016 Brexit referendum.

He stuck with Mr Johnson when he quit two years later over her proposed Brexit deal and worked on his campaign to succeed her as Tory leader.

If he is followed out of No 10 by Mr Cummings, there will be many in Westminster who will not be sorry to see him go.

As Mr Johnson’s top adviser – who was brought in when he became Prime Minister last year – Mr Cummings has been a pivotal figure in Downing Street.

However his abrasive manner and open contempt for MPs has earned him many enemies in Whitehall.

The latest manoeuvrings come amid growing unhappiness among increasing numbers of Conservative MPs at the performance of No 10 and its handling of the coronavirus outbreak.

Sir Geoffrey Clifton-Brown, a senior member of the influential Tory backbench 1922 Committee, said Mr Johnson needed someone who could act as a link with the party in Parliament.

“I think it is essential for the Prime Minister to have a chief of staff. Somebody we can get hold of if we really need to,” he told the PA news agency.

A Labour Party spokesman said: “On the day the UK became the first country in Europe to report 50,000 coronavirus deaths and the public endured another day of 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 incompetence and divided Conservative Government.”