The party took place in late June, less than two weeks after the Queen attended a socially distanced Trooping the Colour – the first time the ceremony had taken place at Windsor since 1895.
During lockdown the Queen and the Duke of Edinburgh were protected at the castle by what became known as the "HMS Bubble".
Several guards will spend time in jail for a lockdown breach. Photo / AP
At the height of the pandemic, her 24 servants were split into two teams of 12 who worked three weeks on and then three weeks off.
They also had to spend a week in quarantine before they got back from leave.
The guardsman were treated as a single household and under the rules they were banned from mixing with other people.
They could not even meet their families to make sure the royals were protected. But at the party they broke the rules by mixing with locals, sources said.
An army source added: "There was never any danger to the Queen and Prince Philip. They would have had absolutely no contact with the royals or members of the royal household."
The other three guardsmen, including a more senior lance sergeant, will be told of their punishment in the coming days.