LONDON — Prime Minister Boris Johnson, defying a storm of criticism, said on Sunday that he would not dismiss his most influential adviser, Dominic Cummings, for breaching Britain’s lockdown rules by driving across the country to visit relatives, even when he was falling ill with the coronavirus.
Mr. Johnson’s decision to stand by his adviser underlines his deep reliance on Mr. Cummings, who was the architect of his election victory last year and the driving force behind his ambitious post-Brexit agenda. But it is unlikely to defuse the uproar over Mr. Cummings’s actions, which critics say send a signal that Britain’s leaders can ignore the rules they impose on others.
The prime minister staunchly defended Mr. Cummings for driving in April to visit his parents in Durham, in the north of England. Mr. Cummings has said there was no other way to get care for his young child after he and his wife began showing symptoms of the virus.
“He followed the instincts of every father and every parent, and I do not mark him down for that,” Mr. Johnson said Sunday at a news briefing. “I believe that in every respect, he has acted responsibly, and legally, and with integrity.”
But Mr. Johnson deflected questions about whether he had known of Mr. Cummings’s travels and muddied the details of the lockdown rules. Rather than offering a coherent defense, the prime minister generally tried to change the subject — in this case, by confirming a previously announced plan to reopen schools on June 1.
The only clear result of Mr. Johnson’s blustery performance is that he has latched himself to Mr. Cummings, a brilliant but polarizing strategist who devised the “Vote Leave” campaign that led to Britain’s departure from the European Union and put Mr. Johnson on the road to Downing Street.
Mr. Johnson’s decision was a rebuke to several lawmakers in his Conservative Party who had declared that Mr. Cummings had to go. They said that for a powerful official to travel at a time when the government was urging people to stay at home — especially those with symptoms of the virus — would undermine efforts to send a strong message about social distancing.
The opposition Labour Party called for an inquiry into Mr. Cummings’s conduct and accused Mr. Johnson of double standards.
“It is an insult to sacrifices made by the British people that Boris Johnson has chosen to take no action against Dominic Cummings,” the Labour leader, Keir Starmer, said in a statement. “The public will be forgiven for thinking there is one rule for the prime minister’s closest adviser and another for the British people.”
Mr. Cummings has been unrepentant. On Saturday, he told reporters he did not care about the optics of the trip and lectured photographers outside his house for not standing far enough apart.
Mr. Cummings said that as he and his wife were both coming down with the virus, they tried to arrange care for their young child at his parents’ house 260 miles from London. While he was ill, the government said, he stayed in separate quarters from his parents and did not leave the house.
But that account came under question after The Observer and the Sunday Mirror reported that Mr. Cummings and his family had been spotted elsewhere on Easter Sunday. The police in Durham said Mr. Cummings’s father had called them asking about security, contradicting an assertion by Downing Street that they had not been in contact with the Cummings family.
Mr. Johnson asserted that some of the allegations about Mr. Cummings’s travels were “palpably false,” but he did not say which ones.
Although Mr. Johnson had few answers about Mr. Cummings, he discussed the government’s move to reopen some classes for younger children in England on June 1. Starting on June 15, there should be “some contact” provided for older children too, Mr. Johnson said.
The push to restart schools has prompted a backlash from teachers’ unions and in some regions where officials believe it is too early. In Scotland, the authorities have decided to keep schools closed until August.
For Mr. Johnson, the Cummings uproar complicates an already messy effort to emerge from the lockdown. There were signs of dissent within the government. Shortly after he spoke, a tweet from the official account of the Civil Service declared: “Arrogant and offensive. Can you imagine having to work with these truth twisters?”
The cabinet office later said the post was unauthorized, had been removed and was being investigated.
From the very start, Mr. Johnson’s instinct was to dig in on behalf of Mr. Cummings. On Saturday, Downing Street marshaled a parade of supporters as cabinet ministers took to Twitter.
“Caring for your wife and child is not a crime,” wrote Michael Gove, a senior cabinet minister for whom Mr. Cummings once worked.
Even Matt Hancock, the health secretary, who has spent weeks urging Britons to comply with the lockdown, argued on Twitter that Mr. Cummings was “entirely right” to have sought child care in the way he did.
This month Mr. Hancock said he was “speechless” that Neil Ferguson, an epidemiologist who advises the government, had breached the lockdown rules by inviting a woman into his apartment. Mr. Ferguson was forced to resign from an influential scientific advisory panel.
But for all the votes of confidence, there was an ominous silence from many Conservative backbench lawmakers. On Sunday, the dam began to break.
Steve Baker, an influential Brexit supporter, urged Mr. Cummings to resign, calling it “intolerable that Boris’s government is losing so much political capital.”
Damian Collins, a lawmaker with whom Mr. Cummings has clashed, also called for him to quit. And other Conservative lawmakers, like Caroline Nokes, were on the receiving end of anger from their constituents.
“There cannot be one rule for most of us and wriggle room for others,” she wrote. “My inbox is rammed with very angry constituents, and I do not blame them.”