No 10 says the PM’s chief aide Dominic Cummings acted in line with coronavirus guidelines when he travelled with Covid-19 symptoms from London to his relatives in Durham during lockdown.
A spokesman said it was “essential” for Mr Cummings to ensure he had childcare while he and his wife were unwell.
The Durham police chief said it was “unwise” for Mr Cummings to travel to County Durham while infected.
Labour has called for a “clear explanation” for Mr Cummings’ actions.
Downing Street said Mr Cummings believed he “behaved reasonably and legally” when he made the 260-mile journey from his London home to Country Durham during the lockdown.
A No 10 spokesman added: “Owing to his wife being infected with suspected coronavirus and the high likelihood that he would himself become unwell, it was essential for Dominic Cummings to ensure his young child could be properly cared for.
“His sister and nieces had volunteered to help so he went to a house near to, but separate, from his extended family in case their help was needed. His sister shopped for the family and left everything outside.
“At no stage was he or his family spoken to by the police about this matter, as is being reported.”
The government has ordered anyone with coronavirus symptoms to self-isolate at home and not leave – even for essential supplies – for seven days.
A Labour spokesman said the government should use the daily Downing Street Covid-19 press conference on Saturday to provide answers over the reports.
A statement added: “The public have made extraordinary sacrifices during this pandemic and the lockdown.
“It cannot be one rule for those who set them and another for the British people.”
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.
The Durham police and crime commissioner Steve White said: “Given the whole ethos of the guidance and regulations was designed to reduce the spread, regardless of reason, by travelling to County Durham when known to be infected was most unwise.
“To beat this crisis we need to be selfless as millions have been.”
Earlier, the Scottish National Party’s Westminster leader Ian Blackford told BBC Radio 4’s Today programme that Mr Cumming’s position was “untenable” and that the prime minister has “serious questions” to answer about the reports.
Meanwhile, Dave Penman, general secretary of the FDA, the union that represents senior civil servants, said the prime minister was responsible for the actions of his chief aide, and called for Mr Johnson to explain reports that Mr Cummings broke lockdown rules.
It comes after other high-profile figures involved in tackling the pandemic have resigned for breaching lockdown restrictions, including Scotland’s former Chief Medical Officer Catherine Calderwood and leading scientist Prof Neil Ferguson.
Dr Calderwood resigned in April after making two trips to her second home during the coronavirus lockdown.
While Prof Ferguson quit as a government adviser on coronavirus after it was reported that a woman he was said to be in a relationship with visited his home in lockdown.
On 30 March, it was reported that Mr Cummings, the former Vote Leave chief who was the architect of the PM’s Brexit strategy, had developed symptoms of the coronavirus and was self-isolating at home.
Durham Police confirmed officers had spoken to the owners of an address after its officers were made aware on 31 March of reports a person had travelled there from London.
A spokesman said that owners of the address confirmed the individual “was present and self-isolating in part of the house”, and officers “explained to the family the guidelines around self-isolation and reiterated the appropriate advice around essential travel”.
Downing Street said that “at no stage” was Mr Cummings or his family spoken to by police about the matter.
“His actions were in line with coronavirus guidelines,” a spokesman said.
Government advice on 31 March was for the public to stay at home and only leave for clearly defined reasons, including to exercise, essential shopping or for medical needs.
At the time – and as remains the case – those with symptoms of coronavirus were told to self-isolate at home and not leave even for essential supplies, if possible, for seven days.
It was and also remains not permitted to leave your house to visit friends and family in their home, while government advice for those aged 70 and over continues to be that they should minimise contact with others outside their household.
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In other developments:
- Police are “wary” on patrol amid a rise in spitting attacks during the coronavirus pandemic
- People arriving in the UK must self-isolate for 14 days from 8 June to help slow the spread of coronavirus, the government has said
- The founder of the UK’s weekly applause for front-line workers has said it should end next Thursday
- A care home in Oxfordshire has told how it locked down early to protect its residents
- And the BBC’s medical correspondent Fergus Walsh has tested positive for coronavirus antibodies