THE head of the civil service has been urged to open an official enquiry into the Dominic Cummings scandal - and act where the Prime Minister won't.

In a letter to the head of the civil service, SNP Westminster leader Ian Blackford urges the Cabinet Office to launch a probe into the senior adviser's lockdown travel.

Failing to do that, the letter claims, could leave Cummings "completely unaccountable" for his actions, which police have said likely constitute a law breach.

In a letter to Cabinet Secretary Sir Mark Sedwill, Blackford says: "Public concern has grown exponentially as a direct result of the Prime Minister’s continued failure to act on this matter.

"This has done serious damage to the functioning of a trustworthy, transparent and accountable government.

"At a time of unprecedented crisis, this matter at the very heart of the UK Government has severely impacted on public trust and undermined vital public health advice.

"Given that serious context, and particularly given the Prime Minister’s failure to act, I believe it falls on you to reconsider your initial reluctance to get involved in this matter."

The letter is released today ahead of the Tuesday return of parliament.

Cummings has insisted he acted within the rules and, lining up to support Johnson's key aide, a clutch of senior Tories have said he did nothing but protect his family.

And Boris Johnson has refused to say any more on the matter or take any action against Cummings.

Meanwhile, more than 1 million people have signed a petition calling on him to go.

Blackford commented: "There is a real sense of indignation that Boris Johnson has refused to take his responsibilities as Prime Minister seriously and failed to remove Mr Cummings from post — severely undermining trust in the UK Government and its public health guidance.

"MPs are still being flooded with messages from our constituents who are flabbergasted that this senior Tory adviser is acting with impunity and getting away with it."

He went on: "Leading public health experts and the police have warned that this scandal has undermined public trust. Polling shows two-thirds think people will be less likely to obey the rules — and there is evidence that they are already being broken as a result.

"No one should be above the rules. If the Prime Minister won't act, then there must be a Cabinet Office inquiry into the rule-breaking and the cover-up, and Parliament must find ways to hold Mr Cummings to account.

"Otherwise people will think this is the establishment protecting itself."