Dominic Cummings travelled hundreds of miles from London to County Durham during the lockdown when he had virus symptoms, the BBC has been told.
A source close to the PM’s chief aide confirmed reports he and his wife went to his parent’s home to self-isolate.
But the BBC’s Laura Kuenssberg said she had been told Mr Cummings did not break government guidance because he and his wife stayed in a separate building.
The story was first reported by the Mirror and Guardian newspapers.
Labour said No 10 needed to provide a “very swift explanation” for Mr Cummings’ actions.
“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,” a spokesman said.
“The British people do not expect there to be one rule for them and another rule for Dominic Cummings.”