Boris Johnson has been urged to launch "a full public inquiry" into allegations that emergency Covid funds have been channelled into political projects favoured by Downing Stret.
In a Commons exchange on Wednesday SNP Westminster chief Ian Blackford said there was evidence cash had gone to conduct polling about the union.
He described the revelations as a "gross misuse of public funds" and accused the government of sanctioning "corrupt campaigning".
Scotland's Herald newspaper first reported that a £560,000 pandemic research contract with a public affairs firm had been extended to cover research on "attitudes to the UK union".
The contract was awarded to Public First, which is run by policy specialists James Frayne and Rachel Wolf, both of whom previously worked with Dominic Cummings and Michael Gove.
"These emergency Covid contracts were supposed to be used for things like PPE for our brave doctors and for nurses fighting Covid," Mr Blackford told the Commons.
"Instead, during the height of this deadly pandemic the Chancellor of the Duchy of Lancaster used these emergency contracts to commission political research - and I quote - on attitudes to the UK union.
"What's worse, he handed these lucrative contracts to long-term friends and former employees. In essence: this was a UK Government contract that sanctioned corrupt campaigning, Prime Minister.
"If the Prime Minister has even a shred of credibility, will he now commit to a full public inquiry on this gross misuse of public funds?"
Boris Johnson told MPs: "I can't think of a better use of public funds than making sure that the whole of the UK fights the Covid pandemic together.
"I believe that the story of these last two years has shown the incalculable value of our union and the strength of our union, and we are better together."
He added that he was "not aware of the contract to which [Mr Blackford] refers" but that: "What I can tell him is that I think that the union and the benefits of the union have been incalculable throughout the Covid pandemic.
"The vaccine rollout ... is of course something, vaccines have been pioneered in Scotland, they've been brewed in Oxford, bottled in Wales, rolled out throughout the UK, and I think it's a tribute to the union that he seeks to undermine."
Earlier this month the High Court ruled that the aware of the half a million pound contract to Public First had been unlawful, after a legal challenge by the Good Law Project.