Westminster leader Ian Blackford said the controversy over Mr Cummings’ 260-mile trip to stay on his parents’ property in Durham while ill with coronavirus symptoms would “cause lasting damage to public confidence” in the government and its public health messages.
It comes as the interim Police and Crime Commissioner for Durham called on the force to investigate any criminality involved in Mr Cummings’ actions.
By noon today, 18 Conservative MPs had publicly called for Mr Johnson’s top adviser to quit or be sacked, with several more calling for an independent investigation.
It remains unclear when the Prime Minister knew of Mr Cumming’s trip to Durham, or whether the former Vote Leave chief made multiple visits, as reported in the Mirror and Guardian newspapers over the weekend.
Eyewitnesses have also been quoted claiming that Mr Cummings was seen at Barnard Castle, 30 miles away from his parents’ farm, where ministers have insisted he remained in isolation.
Downing Street continues to defend the most important figure behind the scenes of the Prime Minister’s administration, with Mr Johnson saying on Sunday that Mr Cummings had acted “responsibly, legally and with integrity” by following his “instincts as a father” in seeking additional support to care for his four year-old son.
Labour leader Sir Keir Starmer has called for a civil service investigation and said that as Prime Minister, he would sack Mr Cummings.
In an early day motion - a symbolic parliamentary device that will not be voted on or debated - Mr Blackford called on Boris Johnson “to seek the immediate resignation of his chief adviser”.
The motion also “notes that other senior public employees have resigned or been removed for similar breaches of lockdown rules” and calls on the Cabinet Secretary to “conduct a swift inquiry into the sanctioning of this breach of lockdown rules and any measures taken within government to restrict full transparency on this matter”.
Mr Blackford MP said: "The Prime Minister's judgement is on now the line - he must stop treating the public as fools.
“There cannot be one rule for Tory advisers and another for everyone else. Dominic Cummings broke the rules and he is now threatening public health. He must go without further delay.”
The SNP MP added: "Public anger is palpable. MPs are receiving hundreds of emails from their constituents and hundreds of thousands have signed a petition calling for Mr Cummings to go.
"Tomorrow we will hold a cross-party meeting for MPs to discuss the joint actions we can take to hold Boris Johnson and Dominic Cummings to account.
"The longer the Prime Minister allows this farce to continue the more the Tory government will lose credibility and respect. He must sack Mr Cummings without further delay."
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