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.
They said there were "reasonable grounds to suspect that an offence or offences may have occurred".
"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," Sir Keir said, as he responded to weeks of Mr Johnson branding him "Captain Hindsight".
The SNP’s Westminster leader Ian Blackford also joined the attacks on the prime minister, saying Mr Johnson was "up to his neck in a slump of Tory sleaze".
"We’ve seen contracts for cronies, texts for tax breaks and cash for curtains," he said.
"The prime minister had dodged these questions all week and he has dodged them again today, but these questions simply are not going to go away."
Mr Blackford asked: "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 the prime minister hit back, telling MPs: "The answer is that I have covered the costs. I have met the requirements that I’ve been obliged to meet in full."
The prime minister – who said no "rules or laws" had been broken in relation to the works on his flat – also claimed the "credulity of the public" was being "strained to breaking point" by Sir Keir’s focus on the refurbishment.
"He goes on and on about wallpaper when I’ve told him umpteen times now, I paid for it," Mr Johnson said.
When asked by Sir Keir to answer a "multiple choice" question on who paid the initial invoice for the works – taxpayers, the Conservative Party, a private donor, or himself – Mr Johnson replied: "I have covered the costs."
When Health Secretary Matt Hancock took part in a Downing Street news conference later in the day, he refused to answer three of the questions about the flat refurbishment and the electoral commission investigation, saying it was meant to be about coronavirus.
He did, however, say he never heard Mr Johnson say "let the bodies pile up" as the prime minister faced pressure to enforce another lockdown last year.
Challenged on his refusal to engage, Mr Hancock said there had been "endless questions in the house of commons" and told a journalist from the Daily Mirror he ought to instead "concentrate on the big things that really matter".
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".
After PMQs, Sir Keir’s spokesman disclosed that the Labour leader’s wedding list was with John Lewis.
"Unlike the prime minister he doesn’t turn his nose down at John Lewis thinking it’s down market," he said.
The spokesman also suggested the fiery exchanges in the House of Commons had seen the prime minister suffer a "Kevin Keegan moment", as he referenced the former Newcastle United manager’s famous rant during the 1996 Premier League title race.
The Electoral Commission investigation comes amid growing questions about how Mr Johnson’s flat refurbishment was paid for.
In leaked emails, it recently 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."
Downing Street said Mr Johnson would be "happy to assist" if the Electoral Commission asks for any information from him during its investigation.
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 of electoral law.
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.
The Cabinet Office said Mr Johnson and Lord Geidt had agreed that he would begin his work by "ascertaining the facts surrounding the refurbishment of the Downing Street flat and advise the prime minister on any further registration of interests that may be needed".
This will be in collaboration with the work of Cabinet Secretary Simon Case, who this week told MPs he is reviewing the funding arrangements of the refurbishment.
But Downing Street said Mr Johnson will remain the "ultimate arbiter" of any investigation into whether he breached the ministerial code.