Boris Johnson joined Downing Street officials for a party at No 10 during the first Covid-19 lockdown – with the prime minister telling one attendee they deserved a drink for “beating back” the virus, The Independent has been told.

It is understood that about 20 civil servants and advisers gathered on 15 May last year for celebratory drinks inside No 10 and its garden. At the time, people from different households were restricted to one-on-one meetings outdoors, with gatherings indoors strictly forbidden.

The gathering took place after the then health secretary Matt Hancock had delivered a televised press conference, noting that 384 coronavirus deaths had been recorded in the previous 24 hours and highlighting a gentle easing of restrictions. The group is understood to have drunk alcohol, including wine, beer and coke mixed with spirits, and eaten pizza, with some staying late into the night.

The revelations – uncovered in a joint investigation by The Independent and The Guardian – are likely to pile pressure on Downing Street, which has come under fire over gatherings in No 10 around Christmas last year.

Drinks were poured at desks as participants huddled chatting just after the press conference finished at around 6pm that evening. It is understood that some staffers carried their alcohol into the Downing Street garden, making the most of the day’s good weather.

The event was characterised as a party by one source in attendance, who said aides had planned in advance to stay on for drinks, with a celebratory mood in the air as restrictions were loosened.

At one stage early on, according to a witness, the prime minister was present for around 15 minutes, during which he told an attendee inside, who had a drink in their hand, that they deserved their beverage for “beating back” the virus.

Only days earlier, on 10 May, Mr Johnson had told the nation in a televised address about plans to ease restrictions and, in a reference to the hardship that the country had faced, stressed that people had “shown the good sense to support those rules overwhelmingly”. Outlining an increase in fines for restriction breaches, he told the nation: “You must obey the rules on social distancing, and to enforce those rules we will increase the fines for the small minority who break them.”

Mr Hancock – who was forced to resign earlier this year after being caught kissing his aide in breach of social distancing guidelines – was present at one point during the event, according to a witness. However, a spokesperson for Mr Hancock said: “These allegations are not true.”

Asked about Mr Johnson’s “beating back” comment, and his presence at the alleged party where officials were drinking and socialising, a No 10 spokesperson said: “In the summer months Downing Street staff regularly use the garden for some meetings. On 15 May 2020 the prime minister held a series of meetings throughout the afternoon, including briefly with the then health and care secretary and his team in the garden following a press conference.

“The prime minister went to his residence shortly after 7pm. A small number of staff required to be in work remained in the Downing Street garden for part of the afternoon and evening.”

A spokesperson for Matt Hancock said: “Matt arrived at Downing Street at 4:43pm and gave the press conference that evening on lifting lockdown measures. After the press conference, which finished at approximately 5:53pm, Matt debriefed his own team, then went to the Downing Street garden to debrief the prime minister. He left Downing Street at 6:32pm and went back to the Department of Health and Social Care.”

There is no suggestion that either Johnson or Hancock drank alcohol themselves, or stayed late.

Mr Hancock had highlighted at the 50-minute press conference, held immediately before the gathering, that people could now meet one other person from another household, outside, for the first time since lockdown was enforced in March 2020 – a change that had come into effect two days earlier.

Mr Hancock described the “shared sacrifice” people had made in observing the stringent measures imposed in the months up to May in order to curb the impact of the virus.

Details have emerged in recent weeks of a series of parties held in No 10 and Whitehall at the end of last year, despite Covid-19 restrictions at the time. They reportedly include a party in No 10 on 18 December, when London was in tier 3, as well as a “packed” leaving do on 27 November, while the country was in its second lockdown. Separately, a Christmas quiz took place on 15 December at which the prime minister virtually hosted a round of questions. A picture emerged showing Mr Johnson sitting next to a colleague wearing a festive hat.

Cabinet secretary Simon Case, the UK’s most senior civil servant, is leading an investigation into the events. Mr Case has been given “a broad remit to investigate anything he thinks should be [investigated], any type of gathering at Downing Street that should be looked at, on any particular day he wants to,” health secretary Sajid Javid told Sky News on Monday.

Meanwhile, another picture emerged in recent days, adding to public anger. The image, published by the Daily Mirror, showed attendees of a festive gathering posing for the camera at Tory HQ on 14 December last year, when measures in London prohibited indoor social mixing. The picture shows elaborate platters of food, with grinning staffers posing alongside the then Conservative London mayoral candidate, Shaun Bailey. Mr Bailey has since apologised for attending, calling it a “serious error of judgement”. He said he gave a speech to thank his team “before leaving shortly afterwards”.