LONDON — Prime Minister Boris Johnson took a breathtaking gamble with his own popularity on Monday, allowing his closest aide to go public with a detailed, yet stubbornly unapologetic, explanation for making a 260-mile journey that broke lockdown rules and ignited a political firestorm in Britain.
In an extraordinary hourlong session with reporters, the aide, Dominic Cummings, admitted to traveling from London to his parents’ home in Durham in late March, shortly before falling ill with the coronavirus, and making a second outing in the region while there, confirming reports that have consumed the British news media for three days.
It was a riveting tableau: a powerful, unelected political adviser seated in shirt sleeves at a table in the garden behind 10 Downing Street offering his version of a deeply personal story that has mushroomed into a national scandal.
For many Britons, it was the first time they had heard Mr. Cummings speak, let alone offer them a glimpse into his most intimate deliberations. He recounted rushing home from Downing Street in March to find his wife ill with symptoms of the virus and of becoming fearful that they might not be able to care for their 4-year-old son.
And yet there were also signs of the Mr. Cummings all too familiar to followers of British politics: the brusque, Svengali-like figure who has become a staple of the British press. He was unrepentant, defensive, and quick to shift the blame to the news media, accusing it of reporting falsely about him, not just in this episode, but over his entire career.
Looking somber, but showing little contrition, Mr. Cummings explained he had left London to secure care for his son in the event that both he and his wife were incapacitated by the coronavirus. Because of his high media profile, he said, he had been “subject to threats and violence” at his home.
“I’m not surprised many people are very angry,” Mr. Cummings said. “I don’t regret what I did; I think what I did was reasonable in these circumstances.”
By now, most prime ministers might have cut loose an aide whose actions prompted accusations of hypocrisy and muddled the government’s messaging as it struggles to deal with a pandemic.
Around an hour after Mr. Cummings spoke, Mr. Johnson tried to put the furor behind him by announcing new measures to ease the lockdown. Among other steps, outdoor markets and car dealerships will be allowed to open on June 1; department stores and small shops will follow on June 15.
Still, the prime minister said he regretted the anger the Cummings episode had stirred up and noted that he did not know in advance about his plans.
“My conclusion is that he acted reasonably,” Mr. Johnson said, adding that “people will have to make their minds up.”
It was not clear whether Mr. Cummings’s account would avert pressure for his resignation.
While he offered a heartfelt portrayal of a family under pressure, he also admitted to having made a visit to a location more than 20 miles from the house where he stayed in Durham — another apparent breach of guidelines. Britons at the time were instructed to leave their homes only for a daily walk or run, but told not to drive anywhere for recreation.
In the part of his account that might prove least convincing for many Britons, Mr. Cummings explained that he had made the trip — to Castle Barnard, a half-hour drive from Durham — to test whether his eyesight, which he said had been impaired by the illness, was good enough for him to make the five-hour drive back to London.
That raised questions about why he was behind the wheel at all.
When Mr. Johnson was pressed on his aide’s explanation, he said he had suffered his own eyesight problems since recovering from the virus.
Perhaps predictably, Mr. Cummings’s performance did not satisfy his critics.
“The British people were looking for at least an apology from Dominic Cummings for breaking the lockdown,” the opposition Labour Party said in a statement. “They got none. The message from this government is clear: It’s one rule for Boris Johnson’s closest adviser, another for everybody else.”
In his iconoclastic style and studied, casual dress, Mr. Cummings somewhat resembles Stephen K. Bannon, the populist bomb-thrower who rescued President Trump’s campaign in 2016 and set about to dismantle what he called the “administrative state” as chief strategist in the White House.
The difference is that Mr. Bannon was quickly forced out. By contrast, Mr. Johnson has stuck with Mr. Cummings despite protests from at least 18 lawmakers from his own Conservative Party, a number of Church of England bishops, opposition lawmakers and members of the public.
Now, some scientists and opposition politicians are warning that the episode risks undermining the credibility of government public health messaging on the pandemic.
Ruthless, effective and deeply polarizing, Mr. Cummings was central to the Leave campaign that persuaded Britons to vote to quit the European Union in 2016 — and proved, ultimately, a pathway to power for Mr. Johnson.
Mr. Cummings had previously worked for a cabinet minister, Michael Gove, then at the education department, and had ruffled feathers there, too. Former Prime Minister David Cameron once described him as a “career psychopath.”
Installed in Downing Street last year at Mr. Johnson’s side, Mr. Cummings was the architect of his uncompromising campaign to “get Brexit done,” helping to secure a landslide election victory in December for the Conservative Party.
“Boris Johnson is not much of a chess player,” said Tim Bale, professor of politics at Queen Mary, University of London. “He tends to think one move at a time, and he needs people able to see two or three moves ahead and to provide his government with a strategic direction it would otherwise lack.”
But the risks are real, both to Mr. Johnson’s political reputation and to the ability of his government to navigate the coronavirus crisis. Even one Conservative lawmaker, Tim Loughton, admitted that many saw “what looks like ‘double standards’ applied to one government official.”
On Monday, the Sun newspaper published a photo of passengers on a crowded plane and offered a quote from an unnamed official: “Everyone seems to have given up on social distancing. We’re calling it the ‘Dominic Cummings Effect.’”
Though the government is gradually lifting lockdown measures, it intends to order those arriving from abroad to quarantine at home. Suspicions that those who make the rules are not following them will hardly help the police who are expected to enforce the new guidelines.
Devi Sridhar, director of the global health governance program at Edinburgh University, said the public had been “remarkably forgiving” about senior officials getting privileged access to testing and medical care. “But extending this to an exemption from lockdown rules feels like a step too far,” she said.
“This is not a left-right issue,” Dr. Sridhar said. “It’s about right and wrong — and government choosing to defend actions of one of its advisers over the public health message it has been pushing for eight weeks, and which is designed to save lives.”