Attorney General Suella Braverman tweeted her support for Mr Cummings on Saturday saying “protecting one’s family is what any good parent does”.
The MP for Fareham also retweeted the official No10 statement which said Boris Johnson’s most senior adviser had behaved “responsibly and legally”.
Shadow home secretary Nick Thomas-Symonds has now called on Ms Braverman to apologise for the intervention.
He alleged her actions undermined the legal independence of her office and should not have been made before the result of the investigation by Durham Constabulary.
— Suella Braverman MP (@SuellaBraverman)May 23, 2020
Yesterday, the Durham police investigation found he did not break lockdown rules by travelling 260 miles to Durham although they said he “might” have made a “minor breach” of the regulations by driving to Barnard Castle to test his eyes.
Mr Thomas-Symonds told BBC Radio 4's Today Programme: "I can't comment precisely on the communication between the Attorney General and Durham Police but what I can say for certain is this - the Attorney General was wrong to be out making public statements about an individual case before even the police had made a public statement, and particularly given her role as superintendent of the Crown Prosecution Service.
"She shouldn't have been commenting on an individual case in those circumstances. That is to misunderstand the role of the Attorney General - to give unvarnished advice to Government without fear or favour and at the bare minimum she should apologise for that."
Asked whether Mr Cummings broke the law during his trip to Durham during lockdown, he added: "The guidelines that were set up, he broke them."
In her tweet on May 23 Ms Braverman said: "“Protecting one’s family is what any good parent does. The @10DowningStreet statement clarifies the situation and it is wholly inappropriate to politicise it.”
Labour’s Shadow Attorney General Lord Falconer has also written to Ms Braverman arguing that her tweet was part of a “politically orchestrated campaign” in support of Mr Cummings and that had breached the principle that her decisions should be free from “political considerations or influence”.
A letter from Ms Braverman, shared by Lord Falconer, said: “There is of course no question of my having offered any public legal view (as you know, law officers do not publish their legal advice much less tweet about it), nor of seeking to pre-empt any formal investigation.
"The purpose of my comments, as I think would be obvious to a fair-minded observer, was simply to support the decision to clarify events. You will understand that I propose to make no further comment.”
The Standard has contacted the Attorney General’s office for comment.