Downing Street staff held 'wine-time Fridays' throughout the pandemic with Boris Johnson regularly witnessing the gatherings.
Sources told the Mirror that the Prime Minister encouraged aides to “let off steam” despite indoor socialising being banned under lockdown rules.
The regular event was so popular that staff even invested in a £142 drinks fridge to keep their bottles of white wine, Prosecco and beer cool.
Reports that staff took a suitcase to a supermarket to stock up on booze ahead of a party on the eve of Prince Philip ’s funeral were not a one-off.
Sources claimed that aides took turns on Fridays to visit the local Tesco Metro in Westminster with a wheely suitcase to fill up the 34-bottle capacity fridge.
An extraordinary picture has emerged of the chiller being delivered through the back door of Downing Street on December 11, 2020.
At the time rules banned two or more people from different households from meeting indoors, unless “reasonably necessary” for work purposes.
Downing Street today apologised to Buckingham Palace after it was revealed more parties were held in No10 the day before Prince Philip’s funeral last Spring.
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Two leaving bashes - one for Mr Johnson’s top spin doctor James Slack and another for the PM’s photographer - took place at Downing Street on April 16.
It was just hours before the Queen sat grieving alone at St George’s Chapel, Windsor.
‘Wine-time Fridays’ were scheduled into the electronic calendars of around 50 No 10 staff between 4pm and 7pm every week.
They are a long-standing Downing Street tradition, including under previous administrations, to help staff wind down at the end of a busy week.
But the event continued many Fridays even after restrictions were introduced to help curb the Covid surge.
It was organised by the No 10 press office but periodically advisers from other parts of the building would join in.
Sources said one regular attendee when tough mixing rules were in place was Captain Steve Higham, then the PM’s defence adviser.
The Royal Navy officer now commands the new aircraft carrier HMS Prince of Wales which is the UK’s largest warship.
The Ministry of Defence declined to comment.
Whitehall enforcer Sue Gray is investigating rules-busting parties.
It has already been suggested that she plans to criticise the “drinking culture” across Downing Street and other Government departments.
Sources said the gatherings on occasion went on as late as midnight with up to two dozen aides drinking wine and beer and playing games like Pictionary.
They were particularly popular from autumn 2020 to the Spring of 2021 when staff were “fatigued” with the tough Covid restrictions, sources said.
However, their popularity waned as the country began opening up and staff no longer had to rely on them for social interaction.
The PM was said to have attended a “handful” of the gatherings at points when indoor socialising was banned under lockdown rules.
These include an event on November 13, 2020, the day Dominic Cummings walked out of No 10, when he stayed for a glass of wine and chatted to the team.
It has previously been reported that the PM gave a “shambolic” speech for departing communications director Lee Cain, with the pair posing with “Get Brexit Done” boxing gloves.
At the time, England was at the height of its second national lockdown with no indoor mixing between households allowed for social reasons.
But in an explosive revelation, No 10 insiders claimed the PM often witnessed ‘wine-time Fridays’ on his way to the Downing Street flat.
One source said: “Boris used to stop by for a chat while they had a drink. It was on the way up to his flat and the door was usually open. He knew about it and encouraged it.“
Another added: “He could see everyone sitting there drinking as he walked up.
“He would come in and say ‘Hello everyone had a hard week? Letting off some steam? Oh great’.
“The idea that he didn’t know there were drinks is total nonsense. If the PM tells you to ‘let off steam’, he’s basically saying this is fine.”
Downing Street’s humiliating apology to Buckingham Palace comes after two leaving dos the night before Prince Philip’s funeral last Spring.
Staff downed alcohol and revellers danced to music blaring from a laptop, with someone sent to a local shop to cram a suitcase with wine.
The partying, which started indoors, became so raucous in the garden that one guest had a go on - and broke - the PM’s son Wilf’s swing, according to the Telegraph.
The Government’s guidance at the time warned: “You must not socialise indoors except with your household or support bubble.
“You can meet outdoors, including in gardens, in groups of six people or two households.”
A Downing Street spokesman said of the leaving celebrations: “It is deeply regrettable that this took place at a time of national mourning and No10
has apologised to the Palace.
“You heard from the PM this week, he’s recognised No10 should be held to the highest standards and take responsibility for things we did not get right.”
He said: “This shows just how seriously Boris Johnson has degraded the office of Prime Minister.
“The Conservatives have let Britain down. An apology isn’t the only thing the Prime Minister should be offering the Palace today - Boris Johnson should do the decent thing and resign.”
Lib Dem leader Sir Ed Davey said: “Boris Johnson should apologise personally to the Queen for the offence he’s caused her and millions around the country mourning for loved ones.
“He should also use that opportunity to officially hand in his resignation.”
The latest Partygate revelations piled pressure on Mr Johnson, who has bunkered down in No10 after a family member tested positive for the disease.
Grieving families who lost relatives to coronavirus blasted Downing Street staff who partied the night before the royal funeral.
Fran Hall, of the Covid-19 Bereaved Families for Justice said: “If your neighbours had behaved like this, you’d have been disgusted.
“For the people running the country to do it and then lie about it shows a complete disdain for the general public.
“We shared the same pain of grieving in isolation as the Queen did - and she must be just as sickened as we are at hearing this.”
Former No10 director of communications Mr Slack, who was appointed deputy editor-in-chief of The Sun newspaper last year, said: “I wish to apologise unreservedly for the anger and hurt caused.
“This event should not have happened at the time that it did. I am deeply sorry, and take full responsibility.”
Mr Johnson is drawing up a list of officials to offer resignations over the Partygate affair in a bid to salvage his premiership, with chief-of-staff Dan Rosenfield and private secretary Martin Reynolds both featuring, The Independent reported.
Dubbed “Operation Save Big Dog” by the PM himself, also includes a plan to highlight Mr Johnson’s achievements in Government after the publication of Ms Gray’s report, expected next week.
Supporters of the PM urged people to wait for Ms Gray’s verdict.
Security Minister Damian Hinds said: “I think it’s important that we have an investigation ongoing into events, plural, and alleged events.”
Tory backbencher Andrew Bridgen became the fifth Tory MP to publicly demand Mr Johnson’s resignation.
“Sadly, the Prime Minister’s position has become untenable,” he wrote in the Telegraph.
“Leadership is not just about the job title, or even making big decisions,
it is equally about having a moral compass, of knowing not just right from left but right from wrong.”
Sutton Coldfield Conservative Association withdrew its support for Mr Johnson, saying the move reflected “local views at the very grassroots levels”.
A No 10 spokesperson said: “There is an ongoing investigation to establish the facts around the nature of gatherings, including attendance, setting and the purpose with reference to adherence to the guidance at the time. The findings will be made public in due course.”