Mr Ross made the concession on Sunday, having called earlier this year for First Minister Nicola Sturgeon to resign over a probe into whether she had broken the ministerial code amid an investigation into alleged sexual harassment by her predecessor Alex Salmond.
And in a separate TV interview, Ms Sturgeon insisted that Thursday’s election was “not an independence referendum”, as opposition parties hit out at reports that she had ordered work to start on a new blueprint for breaking up Britain at the peak of the second wave of the Covid pandemic in Scotland.
An independent inquiry into Ms Sturgeon’s conduct surrounding the Salmond inquiry exonerated her of several charges of breaching the ministerial code. However, a Holyrood committee report found a “fundamental contradiction” in her evidence amounted to a potential breach.
When asked by Mr Marr on Sunday morning if the Prime Minister should resign if he is found to have broken rules surrounding the funding of the refurbishment of his Downing Street flat, Mr Ross replied “of course”.
His comments come as Mr Johnson faces fresh allegations over Tory donors being approached for funding to pay for childcare for the Prime Minister’s baby son Wilfred.
An unidentified Tory MP received a complaint from a supporter who had allegedly been asked to pay for childcare for the one-year-old, according to reports in The Sunday Times.
Mr Ross said: “Of course – I think people expect the highest standards of those in the highest office of the land and that's why I think people are looking at the investigations that are currently ongoing and waiting for the answers to be heard.”
The Scottish Tories leader added: “Nicola Sturgeon was found to have misled MSPs in Parliament by a cross party committee of MSPs, but I think as your earlier commentators say, there are three investigations underway in terms of what the Prime Minister has done and issues around that and I think it's right that we have serious questions answered on all those points.”
The SNP has called for Boris Johnson to publish bank statements and correspondence relating to the renovation of his flat.
SNP Westminster deputy Leader Kirsten Oswald called for Mr Johnson to release all correspondence relating to the allegations, including WhatsApp messages and texts, as media reports also claimed the Tory Party paid £58,000 for the renovation of his flat, on top of £30,000 additional expenditure funded by taxpayers.
The total refurbishment may have cost as much as £200,000, with reports a second invoice was paid by a Tory donor directly with the supplier.
Mr Ross insisted Mr Johnson, who had previously claimed that “wild horses” would not keep him away from the Scottish election campaign, had not avoided Scotland because of fears that his low popularity north of the border could have an adverse effect on the Scottish Tories’ election performance.
He said: “The Prime Minister knows I'm leading the party here in Scotland. It's the Scottish Conservative and Unionist candidates that are on the ballot paper, not the Prime Minister, and it's our manifesto that's going forward to the people of Scotland in just a few days time.”
Mr Ross added: “The reason he's not been to Scotland is because I'm leading the campaign here in Scotland.
"I'm the one taking the fight to the SNP to try and get the next Scottish Parliament laser focused on our recovery and rebuilding, to try and ensure that we've seen investment in our NHS and our police service, in our education systems. All these things that are under threat at the moment, we can actually have a parliament focused on these issues, if we can stop a second SNP majority and a second independence.”
Separate inquiries into how Mr Johnson’s flat redecoration was funded are being carried out by Simon Case, the head of the civil service, as well as Lord Geidt, the independent adviser on ministers’ interests, and the Electoral Commission.
In an interview with the BBC’s The Sunday Show on Sunday morning, Ms Sturgeon insisted the vote on Thursday was not a mandate for her party to start planning for a second Scottish independence referendum, despite reports she had told officials on February 2 that work on the Independence Referendum Bill must ‘commence immediately’ – more than a month before the Scottish Government published the draft legislation.
Dr Penny Curtis, deputy director of elections at the Scottish Government, in a memo to Ms Sturgeon and constitution secretary Mike Russell, wrote on February 2 to ‘”confirm that you are content that work on preparing the draft Bill should start now”. She said that, for it to be published before the election, work “needs to commence imminently”.
On that date three months ago, the First Minister announced 69 more people had died in the pandemic, while schools remained shut, businesses were closed and the country remained in lockdown.
She said: “Opposition politicians can't have it both ways. They can say I should have spent the last year focusing on Covid, which I've done, and then say you should have spent the last year developing the plan for independence.
“Thursday is not an independence referendum, it's not asking people to vote yes or no. When we ask people to make that choice, just as we did in 2014, we will put forward a detailed perspective.”