Rishi Sunak was not invited to the Downing Street party and was not there, a Treasury source has said.
The prime minister is facing growing public outrage after it emerged a gathering was held in Downing Street on 20 May 2020 during the first lockdown.
In an email leaked to ITV News, a senior Number 10 official appears to invite 100 people to a "bring your own booze" event in the garden. At the time, the public were told they were only allowed to meet one other person from another household outdoors.
After days of stonewalling on the issue, Boris Johnson told MPs on Wednesday that he had attended an event in the garden of No 10 - but said he did not know it was a party and only went to thank staff for their work during the pandemic.
The PM's comments have served to further inflame anger in some quarters, with senior Tories - including Scottish Tory leader Douglas Ross - now calling on him to resign.
Few senior ministers have publicly stood by Johnson as they scramble to distance themselves from the increasingly toxic issue, and now the Treasury has confirmed to Yahoo News UK that Sunak was not at the gathering.
Watch: Boris Johnson admits he went to No10 party during lockdown
"The chancellor was not invited and did not attend the event", a Treasury source said.
Yahoo News UK understands that Sunak was in meetings with officials for most of the day. However it is unclear whether he was in his house, Number 11, which is next door - and from which the event would have been visible.
When asked whether there were ministers present at the event, a spokesperson for the PM declined to say on Wednesday – claiming it would be inappropriate to "pre-empt" the investigation.
At the time of the party, the prime minister was encouraging people to call the police on their neighbours if they believed coronavirus legislation was being violated.
Sunak, who is the bookies' favourite to be the next prime minister, has said little in recent weeks over the growing number of Downing Street party allegations.
He was noticeably absent at prime minister's questions on Wednesday, where he usually flanks Johnson, and instead chose to spend his morning in Ilfracombe more than 200 miles away meeting a pharmaceutical company.
A spokesperson for Dominic Raab, justice secretary and deputy prime minister, on Wednesday also put on the record that he was not at the event.
“It’s categorically untrue – he wasn’t invited and didn’t attend,” they said.
The developments come as Johnson faces growing calls to resign from both the opposition and those in his own party.
Labour leader, Sir Keir Starmer, told the PM in parliament on Wednesday that the time had come for him to resign, saying: “The party’s over, prime minister."
Scottish Conservatives leader Douglas Ross, who had previously said the prime minister should resign if he attended, said Johnson should now step down.
Speaking to Sky News, he said: “Regrettably, I have to say his position is no longer tenable."
Ross also said he intends to submit a letter of no confidence to the Conservative party's 1922 Committee, joining a growing number of his party's disgruntled MPs.
It is possible for Conservative MPs to remove Johnson from Number 10 and trigger a leadership contest if enough are submitted and Johnson were to lose a no confidence vote.
Watch: Boris Johnson's 'insulting' apology over Number 10 party 'rubs salt into wounds', COVID victims' families say