Downing Street says the prime minister has not yet received a fixed penalty notice for the event, which would have been his second fine in the Partygate crisis threatening his future.
Mr Johnson has admitted attending the event in May 2020, held during the first lockdown when his own Covid rules had banned both indoor and outdoor social gatherings.
His principal private secretary, Martin Reynolds, is believed to have invited up to 100 people to the “socially distanced” evening drinks, in warm weather.
But Adam Wagner, a human rights barrister and expert on the many changes in coronavirus legislation, cautioned against believing that meant a fine will follow.
“I have said throughout I doubt the PM will get a FPN for this event as he was in his own home/garden,” he tweeted.
Mr Wagner added that “rules at the time (changed 31 May) only penalised if you were outside the home without a reasonable excuse”.
The senior official who sent it was thought to have provided evidence that the prime minister – as alleged by Dominic Cummings – was warned not to go ahead with it.
He was reported to have warned Mr Reynolds to cancel the party, after which Mr Johnson’s close aide “came to his office” and “asked him why the party should be cancelled”.
Mr Cummings claimed Mr Reynolds also told him he would “check with the PM if he’s happy for it to go ahead”, arguing any claim Mr Johnson was not consulted is “not credible.”
The prime minister provoked ridicule by telling the Commons he “believed implicitly that this was a work event”. There was no such permitted event under the Covid rules.
Mr Johnson’s troubles have deepened after he failed to block a Commons inquiry into whether he lied to MPs when he wrongly told them no rules were broken.
One senior Tory has warned the Conservatives will “reap the whirlwind” of public anger over the parties at the ballot box on 5 May.
The former minister Steve Baker – who told the prime minister the “gig’s up” on his premiership – said his constituents are expressing “fury” over the lockdown breaches.
Other top Conservatives are predicting that a post-local elections challenge to Mr Johnson’s leadership is now inevitable as MPs contemplate no end to the controversy.