An email obtained by ITV News, sent by Prime Minister Boris Johnson' s principal private secretary Martin Reynolds, told more than 100 people they should bring their own booze to the socially-distanced event.
It has been alleged that Mr Johnson and his wife Carrie were among the 40 or so to attend the garden event, with picnic food, on May 20, 2020.
The email urged people to join from 6pm to 'make the most of the lovely weather' on what was the hottest day of the year at the time.
But the nation was still in its first full Covid lockdown when the party reportedly took place. Mr Johnson has now been accused of breaching his own rules.
Only two people from separate households were permitted to meet outside in a public place, provided they stayed 2m apart. At the time there was still no mixing indoors, non-essential shops were shut, and pubs and other hospitality businesses remained closed.
Conservative Mr Hollobone, who previously spoke of his constituents' anger at Government parties, told the Northants Telegraph: “The latest revelations, if true, are very disturbing.
"If it is true that lockdown restrictions were not observed within 10 Downing Street when the rest of the country was doing its best to keep indoors away from friends and family then you can readily see why people are so angry about this.
"There cannot be one rule for Her Majesty’s Government and another rule for all the rest of us.
"I hope the inquiry can be concluded as quickly as possible and the findings published so that we can all see who did what, where and when."
The Government will face an urgent question from Labour about the drinks party in the House of Commons today.
It comes after further allegations of 'cheese and wine' events, with one on the same day that KGH staff paid tribute to those who had died with Covid at the hospital.
ITV News also obtained leaked footage of a mock press briefing, where Mr Johnson's then spokeswoman Allegra Stratton was caught laughing and joking about a Christmas party which is also under investigation.
The Cabinet Office inquiry into allegations of various parties and events is currently being carried out by senior civil servant Sue Gray.
The prime minister has refused to comment on allegations because of that inquiry.
Labour leader Sir Keir Starmer told him to "stop lying to the British public".
The Metropolitan Police say they are contact with the Government about the allegations.