Dominic Cummings has alleged there was a lockdown-breaking party in the Downing Street garden five days after Boris Johnson and up to 17 staff were pictured with wine and cheese.

The prime minister’s former top adviser said people were invited to “socially distanced drinks” on 20 May 2020 by a senior Downing Street official. Cummings claimed he warned at the time this appeared to be against the rules but it went ahead regardless.

The investigation into lockdown-breaking gatherings in Downing Street and elsewhere across government led by Sue Gray, a veteran Cabinet Office official, was widened on Friday night to include the drinks party alleged by Cummings, as well as the cheese and wine get-together on 15 May 2020.

The latter was revealed in a photograph published in the Guardian last month, which showed the prime minister, his wife and 17 officials in the Downing Street garden during the first pandemic lockdown.

Sources said it went on late into the evening, although No 10 insists the photograph published showed a work meeting and that the PM went up to his flat with his family shortly after 7pm. Cummings, who was present at that gathering, claimed it did not constitute a “party” or “organised drinks” and denied it was against the rules at the time.

Cummings’ interpretation of the rules has previously been called into question when the Guardian and Mirror revealed he made a journey to Durham with his family while his wife was ill with suspected Covid during lockdown, and a subsequent trip to the beauty spot of Barnard Castle.

The episode provoked widespread public anger and he left his role later that year following a No 10 power struggle. Since then, he has cryptically referred to “parties” in Downing Street and predicted that pictures could emerge.

In his latest lengthy blogpost, Cummings said the alleged event on 20 May was an event organised by invitation as “socially distanced drinks”. He claimed he had warned at the time that it “seemed to be against the rules and should not happen”. He said this warning was in writing and could be uncovered by Gray’s inquiry.

Cummings went on to allege there were other gatherings in 2021, after he left Downing Street, and that Johnson was aware of them.

Since leaving office, Cummings has been a thorn in the side of the prime minister, repeatedly referring to him as a “trolley” on account of him veering around and changing direction when it comes to making decisions.

In the blogpost on Friday, the former aide outed himself as one of the people at the 15 May gathering. Sources have told the Guardian there was a celebratory atmosphere that evening after a press conference. One source described it as a “wine and pizza party” that went on late into the evening.

In a joint investigation between the Guardian and the Independent, a source said the prime minister had told one of those present – drinking inside Downing Street on the same evening – that they deserved their drink for “beating back” the virus.

However, Cummings claimed in his latest blogpost that they were chatting after having meetings which frequently took place in the garden and that he left shortly after it was taken, at about 7.15pm.

After the meeting, he said, “the PM and I continued talking as it broke up. Someone brought a bottle of wine out to the table. It may have been Martin [Reynolds, the prime minister’s principal private secretary] but I think it was the PM himself who went inside as I was packing stuff up and brought out wine. We carried on chatting about Covid, about domestic priorities, and about how to sort out the Cabinet Office which had totally collapsed. Shortly after Carrie joined us.”

He argued that the prime minister’s then fiancee was allowed under the rules to be in the garden in her own home.

At the time, social mixing between households was limited to two people who could only meet outdoors and at a distance of at least 2 metres. Schools were still shut and pubs and restaurants were closed, with strict controls on social mixing.

More people had been allowed to return to their workplaces but guidance said social distancing of 2 metres should be followed at all times and “only absolutely necessary participants should attend meetings and should maintain 2-metre separation throughout”.

Angela Rayner, the Labour deputy leader, has described the “wine and cheese” photograph as “a slap in the face of the British public”, adding: “The prime minister consistently shows us he has no regard for the rules he puts in place for the rest of us. Alleged drinking and partying late into the evening [at No 10] when the rest of us were only recently getting one daily walk.”

Johnson has faced a string of allegations regarding partying and socialising in No 10 while Covid restrictions were in place. He was forced to order a civil service inquiry, though its head, Simon Case, had to step down over allegations of his own Christmas party.

No 10 has been approached for comment.