Boris Johnson faced calls to sack his chief aide Dominic Cummings after it emerged he was investigated by police after breaching lockdown rules.
Mr Cummings drove from London to Durham with his wife and son to stay with his elderly parents after developing symptoms of coronavirus.
Downing Street said at the time that Mr Cummings was "at home" in isolation, when in fact he was more than 260 miles away.
Police in Durham were tipped off by a member of the public about Mr Cummings' presence at his parents' house and explained to the family that lockdown rules - imposed by Mr Johnson days earlier - outlawed such visits.
Opposition parties said Mr Cummings’s position was “completely untenable” and that Mr Johnson now faced a “test of leadership” over the matter, with even Government ministers questioning whether Mr Cummings could now stay in his role.
However, Mr Johnson appeared ready to stand by his long-time ally, with Government sources insisting Mr Cummings and his wife, who was also ill, had taken “what they believed to be the right decision in the interests of their young child”.
Earlier this month Prof Neil Ferguson quit his role as a government scientific adviser after The Telegraph disclosed that he had broken lockdown rules to meet his mistress.
Matt Hancock, the Health Secretary, said at the time that Prof Ferguson’s behaviour had left him “speechless”. Mr Cummings attends meetings of the advisory group from which Prof Ferguson resigned.
The revelation that Mr Cummings broke the Government’s lockdown rules raises serious questions both for him and for Downing Street, which gave journalists the clear impression he was in London despite repeated questions about his whereabouts.
One minister said: “He’s going to have to go. It’s just arrogance.” Another minister said it was "hard to see how [Mr Cummings] can stay based on what we know".
The minister added: "How can any minister ask the public to obey the rules when the adviser closest to the Prime Minister so flagrantly ignores them?"
Mr Cummings, 48, developed coronavirus symptoms at the end of March and had to self-isolate with his wife Mary Wakefield and their young son for 14 days.
On March 31 the Prime Minister’s official spokesman was asked where Mr Cummings was, and said: “I think he’s in touch with No10 but he is at home, he is self-isolating, he has some symptoms.”
In fact, he was in Durham at the time, not at his home in London, as police made clear in a statement following an investigation by the Guardian and Mirror newspapers.
A spokesman for Durham Constabulary said: “On Tuesday, March 31, our officers were made aware of reports that an individual had travelled from London to Durham and was present at an address in the city.
“Officers made contact with the owners of that address who confirmed that the individual in question was present and was self-isolating in part of the house.
“In line with national policing guidance, officers explained to the family the guidelines around self-isolation and reiterated the appropriate advice around essential travel”.
Ms Wakefield later wrote about how the couple had “emerged from quarantine into the almost comical uncertainty of London lockdown”.
According to the Guardian, Mr Cummings was also spotted near the gate of his parents’ home on April 5, five days after the initial complaint to police and the same day that the Prime Minister was admitted to hospital.
The witness, told the Guardian: “I was really annoyed. I thought ‘it’s OK for you to drive all the way up to Durham and escape from London’. I sympathise with him wanting to do that, but other people are not allowed to do that. It’s one rule for Dominic Cummings and one rule for the rest of us.”
At the time, the Government rules on lockdown stated: “You should not be visiting family members who do not live in your home.
“The only exception is if they need help, such as having shopping or medication dropped off.”
Prof Ferguson is not the only government adviser to resign for breaking lockdown. Scotland’s chief medical officer, Catherine Calderwood, quit her role after making two trips to her second home.
But Sir Ed Davey, the acting leader of the Liberal Democrats, said millions of people had made "incredible sacrifices" to stop the spread of the virus, adding: "If Dominic Cummings has broken the guidelines he will have to resign, it is as simple as that."
Ian Blackford, the SNP's Westminster leader, said: "Dominic Cummings' position is completely untenable - he must resign or be sacked."
A Labour Party spokesman said: “If accurate, the Prime Minister’s chief adviser appears to have breached the lockdown rules. The Government’s guidance was very clear: stay at home and no non-essential travel.
"The British people do not expect there to be one rule for them and another rule for Dominic Cummings. Number 10 needs to provide a very swift explanation for his actions.”
As news of Mr Cummings' transgression broke last night, Mr Johnson tweeted a message which said: "Self-isolate and get tested if you have symptoms. Don't risk spreading the virus."
Downing Street was approached for comment.