Jack Hill/WPA Pool/Getty Images Boris Johnson and Queen Elizabeth on June 11, 2021
The gatherings, which were first reported in The Telegraph on Friday, took place on April 16, 2021, while the Queen and the royal family were preparing to lay the Duke of Edinburgh to rest at Windsor Castle.
At the time, the U.K. was in a period of national mourning for Prince Philip, 99, with flags on all government buildings flown at half-mast. Royal mourners had also been politely asked to refrain from leaving flowers near the gates of Windsor Castle and Buckingham Palace to reduce the potential spread of COVID-19.
"It's deeply regrettable that this took place at a time of national mourning," a spokesperson for Prime Minister Boris Johnson told U.K. journalists during their regular morning briefing on Friday. "No.10 has apologized to the palace for that."
Johnson himself was reportedly not present at Downing Street during the events, which were thrown as going away parties for one of the Prime Minister's official photographers and his former director of communications, James Slack.
According to The Telegraph, they involved "excessive alcohol," with one Downing Street staffer being dispatched to a nearby supermarket to fill up a suitcase with bottles of wine.
The timing of the parties is particularly significant because they occurred at a time when the entire population of England was living under "step two" COVID regulations, which outlawed indoor mixing with anyone from outside their direct household or support bubble.
It is because of these regulations that Queen Elizabeth sat alone in St. George's Chapel in Windsor Castle during her husband's funeral, after 73 years of marriage. The scaled-down service also saw Prince Charles, Prince William and the rest of the royal family abide by the U.K. government's advice on social distancing and mask-wearing.
PA Queen Elizabeth sits alone at Prince Philip's funeral
Prince Harry also went into self-isolation ahead of the funeral after traveling to the U.K. from his home in Montecito, California.
The news is just the latest in a string of revelations about lockdown-busting parties inside Downing Street. On Wednesday, this led to calls for Johnson to resign after he personally apologized to Parliament for spending 25 minutes at a "bring your own booze" party at his offices in May 2020.
The gathering, which was first revealed by ITV News on Monday, was organized by his office manager, Principal Private Secretary Martin Reynolds, via an e-mail which invited 100 members of the Downing Street staff to "some socially distanced drinks in the No10 garden" following "an incredibly busy period".
"We thought it would be nice to make the most of the lovely weather," the email added. "Please join us from 6pm and bring your own booze!"
A Buckingham Palace spokesperson told PEOPLE that they have no comment to make about the parties or the Downing Street apology.