Boris Johnson should resign if he is found to have broken the ministerial code, the leader of the Scottish Conservatives has said as the Prime Minister is investigated over renovations to his Downing Street flat.

Douglas Ross said on Sunday that Mr Johnson should “of course” quit if he is found to have breached the code as probes are under way into whether he properly declared any donations for the lavish refurbishments.

The Prime Minister, however, remains the “ultimate arbitrator” of the code and gets the final say on whether he broke the rules, a situation Labour says allows him to be his own “judge and jury” as the opposition calls for reform.

Mr Ross’s comment came amid signs a string of allegations may be damaging the Tories ahead of Thursday’s elections and as fresh claims over donations emerged.

He was asked on the BBC’s Andrew Marr show if Mr Johnson should quit if found to be in breach of the ministerial code, with Mr Ross having previously called for the resignation of SNP leader Nicola Sturgeon if she had broken the rules.

“Of course, I think people expect the highest standards of those in the highest office of the land, that’s why I think people are looking at the investigations that are currently ongoing and waiting for the answers,” Mr Ross said.

The Electoral Commission this week launched an investigation into whether any donations or loans to pay for the refurbishment of his residence in No 11 were properly declared.

But new standards adviser Lord Geidt has also been tasked with reviewing the controversy, in an investigation expected to touch on whether he has breached the ministerial code.

Foreign Secretary Dominic Raab defended Mr Johnson on Sunday but declined to say whether the Prime Minister should resign if he is found to have broken the law by the Electoral Commission.

“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 told Marr.

Mr Raab declined to deny a claim that a second invoice for lavish renovations of the Prime Minister’s residence in No 11 may have been settled with the supplier by a Tory donor.

And he was asked about the suggestion in The Sunday Times that an MP received a complaint from a Tory donor that they were asked to pay for a nanny for Mr Johnson’s one-year-old son Wilfred.

“I have no idea, you don’t have conversations like that with the PM,” he told Sky’s Sophy Ridge On Sunday, as he derided the claim as “tittle-tattle”.

A No 10 spokeswoman said the Prime Minister “has covered the cost of all childcare”, but did not respond when asked if he paid for the original bill himself or had reimbursed somebody else.

Mr Raab backed the Electoral Commission as some backbench Tories called for reform of the watchdog.

“I trust it to look at these things in the right way,” the Cabinet minister said.

As well as pressure over the renovations, Mr Johnson has been forced to deny saying he would rather see “bodies pile high” than impose a third coronavirus lockdown, on top of a lobbying row and allegations of cronyism.

Although earlier polls suggested the “sleaze” allegations were not significantly denting public support for the Tories, two fresh surveys gave evidence to the contrary ahead of the local elections in England and votes for the parliaments in Scotland and Wales.

The Conservatives fell to a five-point lead over Labour, with 42% compared to 37%, according to the Opinium poll of more than 2,000 adults between Wednesday and Friday.

That put the Tories down two points and Labour up four compared to a week earlier, halving the Conservatives’ lead ahead of the elections, in which some 48 million people are eligible to vote.

And in separate polling, Focaldata put Labour on 39%, one point behind the Tories, who previously had a healthy lead, according to The Sunday Times.

Mr Johnson has denied breaking any laws over the refurbishment of his residence and insisted he had paid “personally” for the works.

But he has refused to say whether he received an initial loan from the Conservative Party, as Downing Street launched two separate reviews into the controversy.