Boris Johnson has insisted he "covered the costs" of the refurbishment of his Downing Street flat as a formal investigation was launched into the revamp.
The Electoral Commission on Wednesday announced it would look into whether any transactions related to the works on the prime minister’s flat, above 11 Downing Street, were properly reported, saying there were "reasonable grounds to suspect that an offence or offences may have occurred".
Later, Mr Johnson faced the fury of MPs during Prime Minister’s Questions, with Labour leader Sir Keir Starmer leading the attacks.
"Dodgy contracts, jobs for their mates and cash for access – and who is at the heart of it? The prime minister, Major Sleaze sitting there," he said.
The SNP’s Westminster leader Ian Blackford doubled down, saying Mr Johnson was "up to his neck in a slump of Tory sleaze".
He added: "We’ve seen contracts for cronies, texts for tax breaks and cash for curtains.
"The prime minister had dodged these questions all week and he has dodged them again today… Will the prime minister publish these details [of the funding] today, or is he going to wait until the police come knocking at his door?"
But Mr Johnson hit back and defended the refurbishment, saying he had "conformed in full with the code of conduct".
He told MPs: "The answer is that I have covered the costs. I have met the requirements that I’ve been obliged to meet in full."
Replying to the Labour leader, Mr Johnson said: "He goes on and on about wallpaper when I’ve told him umpteen times now, I paid for it."
The prime minister and his fiancée, Carrie Symonds, are said to have wanted an overhaul of the flat in order to replace what has been termed Theresa May’s "John Lewis furniture nightmare".
In leaked emails, it has emerged that Tory donor Lord Brownlow offered the Conservative Party a £58,000 donation last year.
The money, according to emails seen by the Daily Mail, was to "cover the payments the party has already made on behalf of the soon to be formed ‘Downing Street Trust’".
In the emails to Conservative Party officials from October, the Tory peer was also reported to have mentioned a £15,000 donation.
However, only the £15,000 donation has been listed on Electoral Commission records, with mystery surrounding the other £58,000 sum.
A Conservative Party spokesperson said: "We believe all reportable donations have been transparently and correctly declared and published by the Electoral Commission.
"We will continue to work constructively with the Electoral Commission on this matter.
"While an investigation is ongoing we will not be commenting further."
It has been confirmed that the government has, since last spring, been looking into whether a trust could be established to help fund upgrades of the prime minister’s residence in Downing Street, with Lord Brownlow having agreed to be its chair.
But no such trust currently exists and Mr Johnson is now said to have "personally" paid for the costs of a "wider refurbishment" of the flat he shares with Ms Symonds.
Yet both Number 10 and the Conservative Party have not denied reports that Conservative Campaign Headquarters paid the Cabinet Office to cover initial costs of the works.
Meanwhile, Sky News has reported Downing Street approached former Labour chancellor Lord Alistair Darling to ask for help setting up a trust to oversee the refurbishment of Boris Johnson’s private flat.
Officials wanted Lord Darling to sit on a trust which would cover any renovation costs of the Downing Street estate.
Lord Darling, who lived above Downing Street when he was Gordon Brown’s chancellor, turned down the role.
He is thought to have argued that the government, rather than a trust, should maintain government buildings, and that the project could attract the unwanted attention from donors who might want favours from the prime minister or a peerage.
Announcing their formal investigation on Wednesday, an Electoral Commission spokesperson said: "We have been in contact with the Conservative Party since late March and have conducted an assessment of the information they have provided to us.
"We are now satisfied that there are reasonable grounds to suspect that an offence or offences may have occurred. We will therefore continue this work as a formal investigation to establish whether this is the case.
"The investigation will determine whether any transactions relating to the works at 11 Downing Street fall within the regime regulated by the Commission and whether such funding was reported as required.
"We will provide an update once the investigation is complete. We will not be commenting further until that point."
The Electoral Commission can issue fines of between £200 to a maximum of £20,000 for breaches.
Meanwhile, the prime minister has announced that crossbench peer Lord Geidt – a former private secretary to the Queen – has been appointed as the new independent adviser on ministers’ interests.
The position has been vacant since November after Lord Geidt’s predecessor, Sir Alex Allan, quit the role.
Sir Alex resigned after Mr Johnson overruled his conclusions about Home Secretary Priti Patel’s "bullying" of civil servants.