Dominic Cummings is facing a possible police investigation under health laws over a claim that he breached self-isolation rules in north-east England, after a weekend of mounting pressure on the prime minister to sack his chief adviser. Retired chemistry teacher Robin Lees made a complaint to the police after reporting that he saw Cummings and his family on 12 April walking in the town of Barnard Castle before getting into a car, a joint investigation by the Guardian and Mirror can reveal. Lees’ alleged sighting of Cummings in the town – 30 miles from Durham, where the aide had been self-isolating after developing Covid-19 symptoms – appeared to come while he was still ill, according to his own account. Writing in the Spectator last month, Cummings said: “At the end of March and for the first two weeks of April, I was ill, so we [Cummings and his wife] were both shut in together.” At a Downing Street press conference dominated by questions about Cummings’s movements during the lockdown, Boris Johnson did not dispute that his adviser had made a trip to the Tees Valley beauty spot. Asked directly about the Barnard Castle visit, Johnson said: “When you look at the guidance, when you look at the particular childcare needs that Mr Cummings faced at the time, it was reasonable of him to self-isolate as he did for 14 days or more with his family where he did. I think that was sensible and defensible and I understand it.” Lees said Johnson should have provided clearer answers. In an email to Durham police sent before the Sunday press conference, he alleged a breach of health protection regulations that officers will be obliged to investigate. It said: “At the time, I felt the three people getting in the car might be breaking lockdown rules. However, in light of recent information, I feel that as well as an important breach of the lockdown, there may also have been concerns over Health Protection regulations.” He also provided the number plate of the car he saw Cummings drive away in. The email said: “I assume you are able to view CCTV to ascertain whether this vehicle travelled locally or from further away.” Responding to Johnson’s appearance at the press conference, Lees said: “Asked directly if he visited Barnard Castle, he didn’t answer it, because now I’m totally convinced he can’t. He wasn’t clear whether Cummings had the virus or when he drove to Durham. He wasn’t clear on any details of the 14-day quarantine. He needs to be clear when symptoms started and when they finished.” Durham police have yet to respond to Lees’ email, but the Guardian understands the force is considering whether it needs to take any further action in relation to Cummings. As well as Lees’ complaint, the leader of the Liberal Democrats group on Durham County Council has demanded that the police open an investigation into whether Cummings broke the 2020 Health Protection (Coronavirus) Regulations. Councillor Amanda Hopgood said she had written to chief constable Jo Farrell because there was a clear public interest in establishing the truth and seeing if lockdown regulations had been breached, potentially on multiple occasions. Hopgood said: “I have sent a letter to the chief constable asking them to see if [Cummings] has committed an offence under section 15 of the health protection coronavirus regulations. “I would hope the chief constable would investigate to see if an offence has been committed. There have been so many different versions of what has happened, an independent investigation by the police is needed.” Durham police are now expected to assess the complaints they have received and decide whether further action, such as an investigation, is needed. Among senior police figures there was ridicule of the claim that Cummings’s decision to act on “instinct” justified his actions. One source closely involved in lockdown policing said: “That is a new one on me.” One senior police figure said the rules about not travelling were crystal clear.