Text messages and photos of staff with drinks are said to have revealed the carousing continued until 1am.
Two leaving dos were held on April 16 last year, during the period of national mourning for Prince Philip.
Fresh allegations were made yesterday, including that an office printer was damaged after alcohol was spilt on it.
Wine and spirits with mixers were said to have been served in disposable plastic cups, while pizzas were reportedly ordered in. Some revellers were said to have used a playground slide belonging to Boris Johnson’s son, Wilfred.
It has previously been reported that his swing was damaged as the two events merged in the No 10 garden.
Downing Street apologised to Buckingham Palace when details of the parties emerged last week, while Mr Johnson – who was not present – said he “deeply and bitterly” regretted what happened.
Around 30 people were reported to have attended the parties – one for departing communications director James Slack and the other for a photographer who was leaving.
Covid restrictions at the time banned socialising indoors except with members of the same household or support bubble. It is alleged 13 parties were thrown overall during lockdowns, including Treasury drinks on November 25, 2020.
A Treasury spokesman said staff had come into the office to work on the Spending Review. But a source said: “We have been made aware that a small number of those staff had impromptu drinks around their desks after the event.”
A Christmas party for the Department for Education, hosted by former education secretary Gavin Williamson on December 10, also sparked controversy. A spokesman said: “It would have been better not to have gathered in this way at that particular time.”