The Prime Minister was believed to have attended the event with roughly 40 others in May 2020, before social restrictions ended.
Those who attended described it as a ‘proper booze-up' with ‘long tables packed with drink, crackers, sausage rolls and other picnic food.'
Martin Reynolds, Mr Johnson's primary private secretary, was reported to have coordinated the party.
According to reports, he sent officials with a party invite that said ‘BYOB,' meaning bring your own alcohol.
It's the latest in a string of purported lockdown-breaking occurrences at No 10.
Sue Gray, a senior government official, is investigating the May 20 ‘party'.
The probe is also looking into tales of Christmas gatherings held when London was under tight restrictions.
Unlike prior accounts, Mr Johnson was not believed to have been present for the Christmas activities, except to compere a virtual quiz.
The latest allegations will be made more serious if Mr Johnson attended the gathering on May 20.
Dominic Cummings said he tried to stop the planned ‘socially detached cocktails' because they ‘should not happen'.
English lockdowns against Covid began in March 2020, and it wasn't until June 1 that groups of up to six people could assemble outside.
‘The mentality of absolute disdain for the rules seems to have been installing itself in Downing Street from the very outset of the administration,' said Deputy Labour Leader Angela Rayner.
At a time when much of the country was battling with empty shelves and a total lockdown, it seems No 10 has been hosting parties from the start.
‘Boris Johnson and his gang are playing the country. We need Sue Gray's report quickly, and Boris Johnson must face the repercussions of his actions.'
In a leaked photo, the Prime Minister and staff dined around cheese and wine in a garden on May 15, 2020.
A staff meeting, Mr Cummings claimed in a blog post on Friday, was not ‘illegal or immoral'.
'On Wednesday 20 May, the week after this shot, a senior No 10 official asked visitors to "socially distanced drinks" in the garden,' claimed the Vote Leave veteran.
In an email to Sue Gray, Mr Cummings said he and at least one other special adviser ‘agreed this was against the regulations and should not happen'.
‘No one cared. ‘I was sick and went to bed early that afternoon, yet this occurrence happened,' he said.
No 10 and Mrs Johnson's spokesman have been contacted for comment.