Rishi Sunak accidentally attended Boris Johnson's birthday gathering during lockdown, Yahoo News UK has been told.
According to a Treasury source, the chancellor went to the cabinet room for a COVID strategy committee meeting, and happened upon the celebration on 19 June 2020.
Yahoo News UK understands he was unintentionally present when a birthday cake was served, along with up to 30 other people – including the prime minister's wife, Carrie, and their interior designer, Lulu Lytle.
At the time, indoor gatherings were banned under strict coronavirus legislation, with breaches punishable with fines.
The news comes after the Metropolitan police announced on Tuesday that they are launching an investigation into suspected rule breaking in Downing Street during the pandemic.
The decision follows weeks of criticism against the force for repeatedly declining to investigate the growing number of rule-breaking parties reported to have happened in Number 10.
Watch: Scotland Yard launches investigation into ‘parties’ in Downing Street
Sue Gray, the civil servant in charge of the inquiry into lockdown parties in Number 10, is set to release her report on the parties this week.
It is understood she already knew about the birthday gathering before it was reported on Monday evening.
Johnson himself commissioned the inquiry in December when reports of parties in Downing Street began to emerge.
Gray was appointed as leader of the inquiry after it was discovered that senior civil servant Simon Case - the prime minister's first choice - had attended a lockdown party himself.
On Tuesday there were indications from Number 10 that sections of Gray's report could be delayed amid suggestions that her findings would have to wait until the Met police had concluded their investigation.
The prime minister's spokesperson said: "I have seen reports suggesting that we were seeking to prevent the publication, and that is not accurate."
They added: "There are discussions still ongoing between the investigations team and the police... that still need to be worked through both in relation to what may or may not be published and the ongoing work of both the police and the investigation."
The Metropolitan police have indicated they are not stopping the report being released in full before they conclude their investigation.
At the time of publishing, at least 15 potentially rule breaking parties are reported to have been held during lockdown on Downing Street.
Last week, Downing Street was forced to apologise to Buckingham Palace for a party that was held in Number 10 on the eve of Prince Philip's funeral, where attendees reportedly wheeled suitcases of wine into the building.
Calls for the prime minister's resignation are growing among MPs on both sides of the House.
In the House of Commons on Wednesday, senior Tory MP David Davis told the prime minister "in the name of God, go" shortly after Christian Wakeford, then Tory MP for Bury South, dramatically crossed the floor and joined the Labour party.
And several senior Tory MPs, including Sir Roger Gale and Andrew Bridgen, have said publicly admitted they have submitted letters of no confidence to the party's 1922 Committee - which has the power to remove Conservative party leaders.
Boris Johnson 'welcomes' police investigation into parties