Opposition MPs are calling for Dominic Cummings to resign if he is found to have broken the lockdown guidance after travelling 250 miles from London to Durham when he had Covid-19 symptoms.
The prime minister’s chief aide and his wife, who was also unwell, stayed at his parents’ home while self-isolating.
Labour demanded No 10 provide a “swift explanation” for Mr Cummings’ actions.
A source close to Mr Cummings denied a breach of the coronavirus rules, saying the couple needed childcare help.
They added the couple had stayed in a separate building at the property.
Downing Street declined to comment on Friday night after the story was first reported in the Guardian and the Daily Mirror newspapers.
But a Labour 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.”
The Scottish National Party’s Westminster leader Ian Blackford said Mr Cummings should resign or be dismissed by Mr Johnson and that it was a “key test of leadership” for the prime minister.
Ed Davey, acting leader of the Liberal Democrats, added: “If Dominic Cummings has broken the guidelines he will have to resign, it is as simple as that.”
And former Conservative MP David Liddington, who was de facto deputy PM under Theresa May, told that “There’s clearly serious questions that No 10 are going to have to address not least because the readiness of members of the public to follow government guidance more generally is going to be affected by this sort of story.”