Downing Street staff partied until 1am in a seven-hour drinking session the night before Prince Philip's funeral, The Telegraph can reveal, as new claims emerged.
People were served wine and spirits with mixers in plastic disposable cups, with alcohol at one point getting spilled on an office printer, according to an eyewitness.
A photograph seen by this newspaper shows No 10 staff - some with drinks - gathered in the Downing Street basement, backing up accounts published earlier this month.
Text messages seen by The Telegraph also indicate that attendees were still partying at the centre of government at 1am, having started at around 6pm.
The existence of photographs could become a point of focus for civil servant Sue Gray's inquiry into the alleged lockdown-breaking parties in Downing Street. She is due to release her findings next week.
The new details relate to two leaving events held in Downing Street on April 16, 2021, the night before the Queen sat alone at the funeral of her husband of 73 years.
One event was for James Slack, then Johnson's director of communication, which took place in the press area and later the garden. He has apologised "unreservedly" for the event.
The second, for a departing photographer, was held in the Downing Street basement - the venue for some of the most eye-catching claims. Later the two gatherings merged in the garden.
At the time Britain was in a period of national mourning after the Duke's death. Strict government Covid rules said: "You must not socialise indoors except with your household or support bubble."
Tory MPs and cabinet ministers have expressed their shock at the events that night since they were revealed by The Telegraph. Downing Street has apologised to the Queen.
Sajid Javid, the Health Secretary, this week called the gatherings "wrong in every single way". Some Tory MPs claimed their emergence triggered rebels to submit no confidence letters in Johnson.
The Telegraph can detail new allegations about what took place that evening, which is said to have seen around 30 people in total attend the leaving parties.
In the basement gathering - attended by those marking the photographer's departure and not the event for Slack, which took place elsewhere - music is said to have been played for hours.
At one point wine was spilled over the printer on which the laptop blaring out music was placed, according to an eye-witness.
There were fears it had been broken, but a government source said no equipment was damaged.
Pizzas allegedly handed round
Takeaway was also ordered into Downing Street, it is alleged, with four large pizzas being handed round in the complex's garden late at night.
Another new claim is that some attendees went down the child slide belonging to Wilf Johnson, Boris Johnson's son. It has previously been reported that Wilf's swing was broken that night.
The Prime Minister was not at either event and spent the day at Chequers, his grace-and-favour country house, according to No 10.
The existence of text messages and a photograph of the event - which the newspaper has decided not to reproduce for source protection - could complicate any defence mounted by No 10.
Gray and her team of investigators have already asked some Downing Street officials to hand over their phones, according to a government source, meaning messages could be discovered.
One former Downing Street special adviser said the party in the complex's basement could only have been held there to avoid suspicion.
The basement is accessed through a door at the bottom of the famous Downing Street central staircase, lined with yellow wallpaper and photos of every previous prime minister.
"If you're doing this in the basement, you're definitely doing it to hide the party," the former adviser said.
"It's just unthinkable, because there are so many other function rooms. You don't have to book them out. I never even saw a drinks reception in there when it was raining."
The room itself is small, with photocopiers and adjoining doors to the "garden rooms" - a suite of offices used by No 10 diary secretaries. It also has a small kitchenette used by staff.
Last week a Prime Minister's official spokesman said: "It is deeply regrettable that this took place at a time of national mourning. No 10 has apologised to the palace."