The resignation, confirmed by a person familiar with the matter, brings to an end 17 months of combative and often controversial influence of a small group of pro-Brexit officials over the British government.

Cummings is the second key adviser to Johnson to go in the space of a day, after tensions blew up over the way the prime minister’s inner circle operates. Late on Wednesday, Communications Director Lee Cain — who worked with Cummings and Johnson to deliver the 2016 Brexit referendum vote — announced he was standing down.

U.K. Transport Secretary Grant Shapps downplayed the departure of Cummings. “Advisors come and go,” he said in an interview with Sky News. The government will stay focused on the big issues, including the handling of the pandemic, rather than “who happens to be in and out,” Shapps added.

Cummings has been at the premier’s side since he took power in July 2019, and was the mastermind of the successful Brexit referendum campaign that catapulted Johnson into the front rank of British politics three years earlier. His departure will deprive the premier of his most important adviser and strategist, who has wielded huge influence over all aspects of government policy, from its pandemic response to Brexit and economic reform.

Cain and Cummings were the two closest aides to Johnson in his political team and he will feel their absence. It’s a critical time for the U.K.: with the country in a second national lockdown and the pandemic death rate rising again, Johnson has just a few days left to finalize a Brexit trade deal with the European Union before it’s too late.

With Cain already gone, Cummings’s departure at year end will dovetail with the U.K.’s departure from its post-Brexit transition arrangements.

Divisive Figure

His presence alongside Johnson at the top of government has always been controversial. In the first turbulent months of Johnson’s premiership in 2019, Cummings waged a campaign against anti-Brexit sympathizers inside the governing Conservative Party, forcing some rebels out for good and ripping up political conventions.

Then Cummings helped steer the prime minister toward a historic election victory last December, winning the biggest Tory majority in more than 30 years on his platform to “level up” economically neglected regions of the U.K.

But when the pandemic hit at the start of the year, the Cummings project ran into a wall. All government focus was turned to combating coronavirus and Cummings himself inevitably became part of the story.

In May Johnson put his own authority on the line to defend Cummings, who was accused of breaking lockdown rules by driving more than 250 miles to seek childcare help when the public were being ordered to stay at home.

The premier was so determined to keep Cummings that he appeared in public at press conferences and in parliament to answer questions and repeatedly defend his adviser’s actions.

Cain Connection

Cain announced he was standing down as Johnson’s director of communications in a statement on Wednesday evening. “It was an honor to be asked to serve as the prime minister’s chief of staff,” Cain said, following reports that he had been lined up for the role by Johnson.

It is not clear why Cain did not take up that role. According to one version of events, Cain and Cummings lobbied the prime minister to give Cain that coveted position. But Johnson was unhappy that the appointment was made public in Wednesday’s newspapers before he had reached a final decision, people familiar with the matter said.

It remained unclear throughout Thursday if Cummings would follow Cain out the door. The answer came, in staggered fashion, late on Thursday night.