Sir Keir Starmer has claimed an investigation into the refurbishment of Boris Johnson’s Downing Street flat could be “over in five minutes” if he revealed “who paid for it in the first place”.

The Prime Minister has insisted he has not broken any laws over renovations of his Number 11 residence after the Electoral Commission launched a formal investigation.

The watchdog said there are “reasonable grounds” to suspect an offence may have occurred, to which Downing Street said Mr Johnson would be “happy” to assist the commission with its inquiries into who initially paid for the work.

The Times reports, quoting a Government source, that Downing Street is concerned “there could be a paper trail” at the Conservative Campaign Headquarters (CCHQ).

Investigators from the Electoral Commission can demand documents and information, and could potentially seek a statutory interview with the Prime Minister as part of the process.

Labour leader Sir Keir told ITV’s Peston: “He’s got the Cabinet Secretary doing an investigation, got the Electoral Commission doing an investigation.

“Those investigations could be over in five minutes if the Prime Minister just answered the question, who paid for it in the first place?”

Challenged by the Labour leader over the issue at Prime Minister’s Questions on Wednesday, Mr Johnson said he “personally” paid for the renovations, but refused to answer whether he received an initial loan from the Tory party.

Questions have been mounting over the flat since former aide Dominic Cummings accused Mr Johnson of wanting donors to “secretly pay” for the renovations to the apartment in a “possibly illegal” move.

The Electoral Commission said the investigation will “determine whether any transactions relating” to the renovations “fall within the regime regulated by the commission and whether such funding was reported as required”.

The Conservative Party said it would “continue to work constructively” with the commission.

“We believe all reportable donations have been transparently and correctly declared and published by the Electoral Commission,” a spokesman said.

Prime ministers get a budget of up to £30,000 per year to renovate their Downing Street residency, but newspaper reports have suggested Mr Johnson has spent up to £200,000.

Last week, the Daily Mail published details of an email from Tory peer Lord Brownlow in which he said he was making a £58,000 donation to the party “to cover the payments the party has already made on behalf of the soon-to-be-formed ‘Downing Street Trust’”.

Labour peer Baroness Jay told BBC Newsnight on Wednesday that she had been approached earlier this year by Lord Brownlow, who “asked if I would like to become a trustee on the Downing Street Trust”.

She said that the proposed trust “would be for the public part of Downing Street not the residences”, although nothing formal was ever agreed.

Former private secretary to the Queen Lord Geidt was appointed as the new independent adviser on ministers’ interests on Wednesday, and immediately launched his own investigation into the flat.

The appointment of the new adviser paves the way for the publication of the latest register of ministerial interests, which could contain details of any donations to fund the flat.

However, the Prime Minister’s official spokesman said Mr Johnson will remain the “ultimate arbiter” of whether the ministerial code has been broken, even if the investigation centres on himself.