Labour leader Sir Keir Starmer has called for an investigation into the funding of renovation work on Boris Johnson’s official flat.
The Electoral Commission says it is trying to establish whether any of the spending needs to be examined within its own remit on political donations.
The Daily Mail has reported that Conservative Party funds were used to cover a six-figure bill for the works.
The Conservative Party says donations were correctly declared.
There have been ongoing reports about the cost of the refurbishments to the Downing Street flat, where Mr Johnson lives with his fiancée Carrie Symonds.
Like several of his recent predecessors, the PM is living in the flat above No 11 Downing Street, the chancellor of the exchequer’s official residence.
A source has told the BBC the interior designer Lulu Lytle has been involved in upgrading the flat, which is much larger than the one above No 10.
Mr Johnson and Ms Symonds moved into the four-bedroom apartment back in July 2019, and welcomed their son Wilfred in April 2020.
The Electoral Commission, which is responsible for regulating party donations, says it is in contact with the Conservatives over the funding of the flat.
A spokesperson said: “Discussions with the Conservative Party continue as we work to establish whether any sums relating to the works at 11 Downing Street fall within the regime regulated by the Commission, and therefore need to be reported and subsequently published.
“The party is working with us on this.”
The Conservative Party has previously said that all “reportable donations” are “correctly declared to the Electoral Commission, published by them and comply fully with the law”.
The party said “gifts and benefits received in a ministerial capacity” are declared in government transparency returns.
The PM’s press secretary has previously insisted that no party funds were being used to pay for any refurbishments to the Downing Street estate.
Asked about reports that Cabinet Secretary Simon Case is investigating the issue, Sir Keir said the matter “should be investigated,” although he did not specify the form such an investigation should take, or who should carry it out.
Asked about Mr Case’s reported involvement on Friday, the PM’s spokesman told reporters: “Maintenance of Downing Street as a working building is overseen by the Cabinet Office.
“As part of that, you’d expect Cabinet Office officials to be involved.”
The spokesman added money spent on any refurbishment would be published in the Cabinet Office’s annual report, which could be expected in the summer.