Labour has accused Boris Johnson of potentially misleading parliament over his knowledge of Downing Street parties, as more details came to light about a quiz he hosted in No 10 last year.

The prime minister came under further pressure on Sunday when a picture emerged of him in a room with aides who wore tinsel and a Santa hat. He read quiz questions to staffers tuning in from home but also groups of officials who were elsewhere in the building, crowded around laptop screens without social distancing, and drinking alcohol. Some staff stayed on afterwards, continuing to socialise.

London was in tier 2 at the time, meaning that while working together was permitted, mixing between households for social reasons was banned, including Christmas events.

Two sources who attended the event gave the Guardian details that cast doubt on Johnson’s insistence at the last prime minister’s questions that there were no Christmas parties or rules broken.

The insiders said Johnson told people to enjoy themselves at the event on 15 December, and that his private office had their own quiz team. Dan Rosenfield, the prime minister’s incoming chief of staff, walked around some teams that dialled in from different rooms in No 10 to introduce himself.

“It wasn’t a huge event and it wasn’t actually a risk to people,” said one attender, who pointed out that No 10 staff were being tested daily when they went into work. But they added: “I think it’s morally wrong and do feel bad about it.”

Angela Rayner, Labour’s deputy leader, accused Johnson of potentially misleading MPs by denying any knowledge of rules being broken when he was asked about the Christmas parties scandal in parliament on Wednesday.

She said: “It appears that Boris Johnson lied to the country and broke the law. It is increasingly clear that the prime minister presided over a culture of ignoring the rules that he told everyone else to follow. It’s time to fess up.”

Bereaved family members have written to the Metropolitan police urging them to investigate the gathering and other parties in Downing Street, with Scotland Yard told to “stop turning a blind eye” to alleged rule-breaking.

Jo Goodman, a co-founder of Covid-19 Bereaved Families for Justice who lost her father, Stuart Goodman, to the virus in April 2020, said the images of Johnson at the quiz “put to the sword the notion that no rules have been broken at Downing Street last year”.

She said: “It is unclear to families who have lost loved ones why evidence like this is not being utilised and that it seems no investigation is taking place by the Metropolitan police, despite numerous requests.”

Jolyon Maugham QC, the founder of the Good Law Project, said the organisation planned to push ahead with a judicial review of the police if they did not open an investigation, which was “difficult but not impossible”.

The picture of the prime minister hosting a quiz for Downing Street staff last Christmas “directly implicates” Johnson in an “undeniable and no longer denied” way, said Maugham.

However, the education secretary, Nadhim Zahawi, dismissed the leaked photo, suggesting he thought it showed that the reports of parties held in No 10 in defiance of lockdown and tier restrictions were overhyped.