Dominic Cummings, British Prime Minister Boris Johnson's chief adviser, is facing a possible police investigation under after he breached coronavirus isolation rules in north-east England.
And pressure is mounting on Mr Johnson to sack Mr Cummings, who was isolating himself with symptoms of Covid-19.
A joint investigation by The Guardian and The Mirror newspapers on Sunday revealed that Robin Lees, a retired chemistry teacher, filed a police complaint after he said he saw Mr Cummings and his family on April 12 walking in the town of Barnard Castle.
Mr Johnson staunchly defended his chief adviser on Sunday following claims that the aide broke the government’s lockdown rules.
At least eight MPs called for Mr Cummings to quit after it emerged that he drove 400 kilometres from his London home to seek support from relatives for his four-year-old son after his wife contracted Covid-19.
Mr Johnson said Mr Cummings acted “responsibly, legally and with integrity" with the aim of stopping the spread of the virus.
"I totally get why people might feel so confused and so offended by the idea that it was one thing for people here and another thing for others," he said.
But Mr Johnson faced an uncomfortable 35 minutes as he was repeatedly questioned about the incident and did not answer questions about why Mr Cummings did not heed the government's isolation rules.
He declined to say when he learnt about the trip or if he had approved it.
The leader of the opposition, Sir Keir Starmer, said in a tweet that it was an "insult" that Mr Johnson had failed to take action against Mr Cummings.
Mr Cummings, who led the main campaign to leave the EU, has become a trusted aide to the prime minister, although his abrasive manner has led to clashes with other members of the Conservative party.
Reports of a second lockdown-defying trip to the north of England were denied by the leadership, and Mr Johnson failed to respond to questions about it.
Despite his attempt to dampen the controversy, questions remain about the episode and the prime minister's judgment in backing Mr Cummings.
The adviser spent hours at 10 Downing Street on Sunday explaining the circumstances around the trip in late March.
In a change of plan, Mr Johnson fronted the daily news conference on the government’s Covid-19 strategy to announce that he would stand by Mr Cummings.
“I think he followed the instincts of every father and every parent,” he said. “I do not mark him down for that.”
Criticisms of Mr Cummings came from MPs who had been allies with the aide during the campaign to take the UK out of the EU.
“I think Dominic Cummings staying weakens government,” MP Peter Bone told Sky News.
“Dominic Cummings going strengthens government. Therefore, that’s the right thing to do.
“We’re in the middle of a national emergency, so let’s end this argument now.”
Fellow pro-Brexit MP Steve Baker said that the adviser’s actions had undermined the government’s message and that was hurting Mr Johnson.
Nicola Sturgeon, the First Minister of Scotland, also called for Mr Cummings to quit.
Her chief medical adviser, Catherine Calderwood, resigned in April after making two trips to her second home.
“I know it is tough to lose a trusted adviser at the height of crisis, but when it’s a choice of that or integrity of vital public health advice, the latter must come first,” Ms Sturgeon wrote on Twitter.
“That’s the judgment I, and to her credit Catherine Calderwood, reached. The PM and Cummings should do likewise.”
Mr Johnson had sought to shift the spotlight to plans for lifting some coronavirus restrictions after the deaths of more than 36,000 people in Britain, the second highest reported death toll in the world, behind the US.
Proposals to reopen schools for some children in England from June 1 have run into difficulties because of opposition from the teaching profession and doctors.