Sir Keir Starmer has called for an investigation into renovation work on Boris Johnson’s official flat.
The Electoral Commission says it is trying to establish whether money being funneled into the project needs to be considered as political donations.
There have been ongoing reports about the cost of the refurbishments to the Downing Street flat, where Johnson lives with his fiancée Carrie Symonds.
Johnson is currently living in the flat above No 11, which is the chancellor’s official residence.
The Conservative Party says donations were correctly declared.
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.
Johnson and 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”.
Johnson’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 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.”
One government minister said Johnson had met the costs of refurbishing his official Downing Street flat out of his own pocket.
In answer to a House of Lords written question, Cabinet Office minister Lord True said: “Audited figures of spending from the allowance for the 2020-21 financial year are not yet available; works have taken place by long-standing Downing Street contractors on painting, sanding and floorboards.
“Any costs of wider refurbishment in this year have been met by the prime minister personally.”
Lord True added that the government was “considering the merits” of whether works on parts or all of the Downing Street estate could be funded by a trust.