Partygate could have been avoided if the police had properly investigated alleged lockdown breaches by Dominic Cummings, a former regional chief prosecutor has claimed.

Nazir Afzal, the crown prosecutor for north-west England until 2015, also expressed exasperation at fresh revelations about the way Durham police looked into Cummings’ journeys to Durham and Barnard Castle after they were exposed by the Guardian and the Mirror in May 2020.

Last month Durham’s chief constable, Jo Farrell, admitted that her officers did not interview Cummings as part of their three-day investigation into the lockdown movements of Boris Johnson’s then chief adviser.

She told the Mirror that the force treated Cummings’s televised press conference in Downing Street’s rose garden as a witness statement about his behaviour instead.

In May 2021, Cummings admitted to MPs that he did not tell the whole truth in the rose garden. “I should probably explain some things about this that were not put into the public domain at the time in the rose garden,” he told a joint session of two select committees.

Speaking to the Guardian, Afzal said: “The police relied on an account which the suspect himself has deviated from subsequently. That in itself makes it likely that the police investigation was flawed.”

Afzal’s lawyers, Hodge Jones & Allen, have written to the Metropolitan police and Durham constabulary urging them to share any “lessons learned” about their handling of Cummings’ behaviour in the light of the more thorough investigation that officers eventually conducted into Partygate.

And they asked whether the Met still stood by its decision not to investigate Cummings leaving London for Durham when he had suspected Covid.

Afzal said: “Had the police promptly and robustly investigated those responsible for setting the rules, but seemingly breaching the rules, Partygate might not have happened.

“In relation to Dominic Cummings, then a central player within Downing Street, Durham police could and should have done so at the time of the breaches and when presented with the evidence and complaints.

“Such action would have reinforced the message to the prime minister and those around him, that the Covid regulations, which they created and encouraged others to follow, should be observed, both in spirit and by the letter.”

After a three-day investigation in May 2020, Durham police concluded that Cummings’ infamous journey to Barnard Castle probably amounted to a “minor” breach of the rules.

But the force took no further action and made no finding on Cummings’ decision to leave London because the investigation was confined to County Durham. It also said there was insufficient evidence that Cummings made a second trip to Durham on 19 April 2020 despite taking statements from nurse practitioner Clare Edwards and her husband, Dave, claiming they had seen him in Durham that day.

In February 2021 Durham police rejected a 225-page dossier submitted by Afzal’s lawyers alleging that Cummings had breached the Covid rules multiple times.

It also alleged that Cummings perverted the course of justice in his account of his journey to Barnard Castle on 12 April and his denial of a claim that he made a second lockdown trip to Durham around 19 April.

Last May, Cummings again insisted to MPs that it was “false” to claim he made a second trip to Durham. But he appeared to concede they did leave London a second time because of security concerns: “It is true that I moved my family out again.”

Asked at the time to explain how four people claimed to have seen him in and around Durham’s Houghall Woods on the morning of 19 April, Cummings told the Guardian: “I did not go back to Durham on [the] 19th. I was in London on [the] 19th and with my son on Hampstead Heath, and phone data proves it.”

The Guardian has seen a photograph taken at 3.31pm that day appearing to show Cummings on Hampstead Heath with his family. It also established that it was possible to drive from Durham to London in under four hours during lockdown.

Explaining Durham’s decision not to interview Cummings, Farrell told the Mirror: “I remember thinking, as he talked, that it would be the account of the circumstances.

“From a proportionality point of view, there was an extensive and well-documented narrative about what he had done and where he had been. We used that and information about the decision-making around it.”

The Met told Afzal’s lawyers that it held no information on lessons learned from the Cummings case and referred the issue to Durham police. Durham police have yet to respond, but Farrell told the Mirror she was “confident and satisfied the decisions we took then will stand the test of time”.