A man who claims he saw someone who "looked like" Dominic Cummings on a family walk 25 miles outside of Durham during lockdown has filed a complaint to police.
Retired chemistry teacher Robin Lees wrote down what he described as a "distinctive" number plate.
Sky News can confirm the registration number is that of a car Mr Cummings has got into in the past.
Mr Lees has contacted Durham Constabulary by email to file an official complaint regarding his alleged sighting of Mr Cummings.
Boris Johnson's top adviser had already admitted that he drove 260 miles from London to Durham with his wife and young son in late March to self-isolate at a family property.
The government defended Mr Cummings by saying the trip was necessary because both he and his wife had coronavirus symptoms and they needed family to help with childcare for their four-year-old son.
But in new allegations published on Saturday night, he was reportedly spotted in Houghall Woods near his parents' Durham home on 19 April.
A week earlier, he was allegedly seen on a walk with his family in Barnard Castle, 25 miles away from the city.
However, no photographic evidence has been published, and the reports are based on the testimonies of two eyewitnesses.
One of them, Mr Lees, told Sky News he spotted Mr Cummings and his family walking by the River Tees near Barnard Castle.
"I saw somebody that looked like him on the 12 April and it just looked strange, as at that time there were not that many people out," he said.
Mr Lees says he has a "photographic memory".
Councillor Amanda Hopgood, leader of the Liberal Democrat opposition on Durham County Council, said she has written to Durham Constabulary Chief Constable Jo Farrell after "a number" of locals reported seeing Mr Cummings in the city and county "on a number of occassions during April and May".
She said she has asked police to investigate whether Mr Johnson's aide "may have committed an offence" under the new coronavirus health protection regulation law.
Sir Roger Gale, MP for North Thanet, who has joined the chorus of Conservative backbenchers calling for the PM to sack his top aide, told Sky News Mr Cummings's political career was "dead in the water" if the number plate Mr Lees saw matched his registration.
During an interview on Sophy Ridge On Sunday, transport secretary Grant Shapps said claims Mr Cummings broke the lockdown twice are "completely untrue", adding that the PM's most senior adviser had stayed put in the capital after returning on 14 April.
He told Sophy Ridge: "I certainly know that the first one you mention, of travelling back up (to Durham), I know that is not true."
The prime minister doubled down on his own defence of his top aide at Sunday's daily briefing, saying Mr Cummings had acted "responsibly, legally and with integrity".