Boris Johnson should resign if he is found to have broken government rules over his flat renovations, the Scottish Conservative leader has said.
Douglas Ross told the BBC that people expected the highest standards of those in the highest office of the land.
But the foreign secretary, Dominic Raab, insisted the prime minister had followed the rules and the newspaper stories were "gossip".
Labour said people needed to know who the prime minister was beholden to.
Mr Johnson has insisted he paid for a refurbishment of the Downing Street flat himself, but there are still questions over whether the costs were initially covered by party donors.
On Wednesday, the Electoral Commission - the watchdog overseeing political finances - announced an investigation into the Conservative Party's compliance with laws on political donations, having said there were "reasonable grounds to suspect that an offence or offences may have occurred".
The government's top civil servant Simon Case and the prime minister's newly appointed adviser on ministers' interests Lord Geidt are also looking into the issue.
Asked by the BBC's Andrew Marr whether Mr Johnson should resign if he was found to have broken the ministerial code, Mr Ross said "of course".
But he added he would wait for the answers from the three separate investigations that are taking place.
Mr Ross has previously called on Nicola Sturgeon, Scotland's first minister and leader of the SNP, to step down after a majority of MSPs on a committee accused her of misleading their inquiry into the Scottish government's handling of sexual harassment complaints made against former First Minister Alex Salmond.
Mr Raab told the BBC's Andrew Marr he would not "speculate on what the outcome of the various different reviews are".
"I think the right thing for me to do is respect the integrity of those reviews and let them run their course rather than commenting on what may or may not be found at the end of it," he added.
He added he trusted the Electoral Commission "to look at these things in the right way".
Mr Raab also told Sky's Sophy Ridge the prime minister would "engage fully" with inquiries into who initially paid for the Downing Street renovations.
And he said he had "no idea" whether Conservative donors were asked to help pay for the prime minister's childcare costs.
The Sunday Times has reported that supporters of the party had been asked to foot the bill for a nanny for Mr Johnson's son.
The foreign secretary dismissed the report as "tittle tattle", telling Sky News: "I can't comment on every little bit of gossip that's in the newspapers."
Responding to the story, Downing Street said: "The prime minister has covered the cost of all childcare."
Also appearing on Sky News, shadow foreign secretary Lisa Nandy said: "We need to know who the prime minister is beholden to, we need to know what he has promised in return."
"There's an arrogance at the heart of this that he seems to believe that we don't deserve to know the truth about what goes on in government."
The prime minister has also received criticism from the former Commons Speaker John Bercow.
Talking to BBC Radio 4's Broadcasting House programme, Mr Bercow said of the flat row: "First of all we don't know who paid the initial bill because the prime minister and his acolytes persisted with the refrain that he ended up picking up the bill.
"That is both true and irrelevant - we know that, we've heard it, we've got it... the point is who initially did so creating either the fact or the appearance of indebtedness and an obligation on the prime minister in the future."
He also accused the prime minister of demonstrating "an insouciant, a flippant, a casual disregard for the accuracy of what he says".
The SNP's Westminster deputy leader Kirsten Oswald said Mr Johnson should publish his bank statements and all relevant documents relating to the flat renovations and donor-funded income and expenditure "to ensure full transparency and assist inquiries and criminal investigations".
Lorna Slater, co-leader of the Scottish Greens, has said she believes the UK political system is "broken" and politicians should be held accountable for their decisions in the midst of the pandemic and the climate crisis.
Scottish Lib Dem MP Christine Jardine said the prime minister was being "rightly scrutinised" but the SNP was not in a position to criticise, following the inquiry into the Scottish government's handling of complaints made against Alex Salmond.