The PM’s chief aide Dominic Cummings is facing calls to resign after it emerged he travelled from London to his parents’ home in Durham with coronavirus symptoms during lockdown.
Mr Cummings and his wife, who was also unwell, stayed in Durham, while self-isolating.
Labour demanded No 10 provided 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 told the BBC’s political editor Laura Kuenssberg that the couple had stayed in a separate building at the property.
Downing Street declined to comment on Friday night, after the Guardian and the Daily Mirror newspapers first reported Mr Cummings had made the more than 260-mile journey.
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The government has ordered anyone with coronavirus symptoms to self-isolate at home and not leave – even for essential supplies – for seven days.
Meanwhile, Prime Minister Boris Johnson said in March that children should not be left with older grandparents or older relatives “who may be particularly vulnerable or fall into some of the vulnerable groups” .
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.”
Mr Cummings has attended meetings of the Scientific Advisory Group for Emergencies (Sage) – the scientific body which gives independent advice that shapes the government’s coronavirus response.