Dominic Cummings, caricatured by British tabloids as a Svengali-like adviser to Johnson, was the leading strategist behind the 2016 Brexit campaign and Johnson's landslide election win in 2019. He was quizzed Wednesday by British lawmakers on the lessons learned from the pandemic.
"The truth is that senior ministers, senior officials, senior advisers like me, fell disastrously short of the standards that the public has a right to expect of its government in a crisis like this," he said in testimony offered to Parliament's Science and Health Select Committee.
"When the public needed us most, the government failed," he said, adding that people "died unnecessarily" as a result.
Cummings delivered excoriating allegations of bad decisions and false assumptions made by Johnson's government, describing scenes of chaos in government as "like an out-of-control movie."
He said that in early 2020, the U.K. government "was not operating on a war footing on this in February in any way, shape or form. Lots of people were literally skiing."
Cummings said Johnson initially regarded the pandemic as "just a scare story" and said the prime minister even suggested at one point being injected live on TV with the virus to reassure the public.
Johnson swiftly rejected the criticism from his former adviser.
He told Parliament later Wednesday that "none of the decisions have been easy" and that he did not accept Cummings' accusation that government inaction led to unnecessary deaths.
"To deal with a pandemic on this scale has been appallingly difficult, and we have at every stage tried to minimize loss of life ... and we have followed the best scientific advice that we can," he said.
The U.K. has recorded almost 128,000 coronavirus deaths, among the highest in Europe. A series of lockdowns have periodically shut down most of the economy, schools and workplaces.
However, a mass vaccination drive, which started in December has brought infections and fatalities down sharply and has been widely lauded as a success.
Cummings — who was played by the actor Benedict Cumberbatch in a 2019 movie about Brexit — is a self-styled political disrupter who has long expressed contempt for the civil service, politicians and much of the media.
One of the architects of the successful campaign to take Britain out of the European Union, he was appointed a top adviser when Johnson became prime minister in 2019.
Cummings was himself thrust into the spotlight during the pandemic when newspapers revealed he had driven 250 miles (400 kilometers) across the country after contracting Covid-19 in May 2020, despite a nationwide stay-at-home order.
His defense — that he was seeking child care help from relatives in case he got sick — rang hollow to many Britons who had made sacrifices and endured isolation to follow the rules.
Johnson resisted calls to fire Cummings for flouting rules the government had imposed on the rest of the country.
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Cummings, nonetheless, left his job in November. Images of him carrying a box of personal belongings as he walked away from 10 Downing Street made the front pages of many British newspapers.
In recent days, he has used Twitter to direct a torrent of criticism at his former employer, accusing Johnson of overseeing a chaotic government.
In turn, Johnson's Conservatives accuse Cummings of glossing over the fact that he was one of the most powerful people in the government when key decisions were being made.
During Wednesday's testimony, Cummings apologized for not doing more to change the government's strategy and raise the alarm on Covid-19 sooner.
"I'm terribly sorry," he said. Adding, "it was completely crazy that I should have been in such a senior position."