Families who lost loved ones during the pandemic have said they are "sickened" by a No 10 Christmas party held during last year's Covid restrictions.
The party took place on 18 December, with a source telling the BBC "several dozen" people attended.
But the Covid restrictions operating at the time banned such events.
Boris Johnson - who was not at the party - said no Covid rules were broken, but No 10 has refused to explain how party-goers complied.
Asked by reporters on Thursday why he would not tell the public what happened at the party, the PM replied: "Because I have told you."
Instead, Mr Johnson tried to focus on this Christmas, saying the government was taking a "balanced and proportionate approach to the risk" and "people shouldn't be cancelling things", like parties and nativities, because of concerns about the Omicron variant.
Labour leader Sir Keir Starmer said last year's event in No 10 showed it was "one rule for them and another rule for everybody else", adding that it sent "completely the wrong message" to the public.
The Covid-19 Bereaved Families for Justice group have called for an apology from Mr Johnson.
One of its spokespeople, Safiah Ngah, told the BBC: "My Dad died in February from Covid-19, despite being in good health. The last Christmas period is sadly one I will never forget.
"One in 20 people in my borough had Covid-19 and my family were desperately trying to do what we could to keep each other safe. Unfortunately it wasn't enough.
"To think that just a few miles away, No 10 was throwing a 'Christmas Party', with no care for the rules they had set, is sickening."
The party in Downing Street last December was first reported by the Daily Mirror on Tuesday.
The newspaper said last year's official Downing Street Christmas party was cancelled due to restrictions, but staff held an unofficial gathering.
A source who attended later told the BBC that party games were played, food and drink were served, and the party went on past midnight.
The event took place two days after London went into Tier 3 lockdown restrictions on 16 December, meaning people were not allowed to mix indoors with anyone outside their household or support bubble.
People were allowed to gather if it was reasonably necessary for work purposes, but that would not have included holding a party.
At a Downing Street press conference on 16 December, Mr Johnson said: "This Christmas it's vital that everyone exercises the greatest possible personal responsibility."
What were the rules on office parties in December 2020?
Any party at Downing Street would have breached the government's guidelines at the time.
Its guidance for the Christmas period specifically said: "Although there are exemptions for work purposes, you must not have a work Christmas lunch or party, where that is a primarily social activity and is not otherwise permitted by the rules in your tier."
The other question is whether it would have been illegal.
London at the time was in Tier 3 and the law banned gatherings of two or more people indoors unless it was "reasonably necessary" for work.
There was also a specific prohibition on organising an indoor gathering of more than 30 people.
But there may have theoretically been a loophole for government buildings like 10 Downing Street because of the 1984 legislation used to bring in the tier system, according to Adam Wagner - a barrister at Doughty Street Chambers and an expert on Covid regulations.
Another barrister, Charles Holland from Trinity Chambers, tweeted that there would have needed to be an agreement with Westminster City Council for the regulations to apply in Downing Street.
On 21 December, restrictions were toughened to Tier 4 measures in London, and on 6 January, the whole of England went into its third full lockdown.
At Prime Minister's Questions, Labour's Sir Keir accused Mr Johnson of "taking the British public for fools" by flouting rules he had put in place.
But the PM insisted all Covid rules had been followed in Downing Street.
Although the PM did not attend the party on 18 December, the Mirror also reported he gave a speech at a "packed leaving do" for an aide on 27 November, when the country was in its second lockdown and socialising was banned.
Labour's deputy leader, Angela Rayner, wrote to Cabinet Secretary Simon Case on Thursday, saying she found it "difficult to understand" how either event could have complied with regulations.
She accused the government of "undermining public health messaging", and asked Mr Case if he would be conducting an inquiry into the events, as well as if he would refer any potential breach of the Covid regulations by the PM to the Metropolitan Police.
Sir Keir said the public wanted a government "who sticks by the rules", adding: "Last year, when the government was telling the country to lockdown and not meet their loved ones, they were partying in Downing Street. this year we cannot see a repeat of that.
"Whatever the rules are, I want to see the government actually obeying the rules this year. Lots of people feel pretty stunned about last year."
'Shame on them'
Reports about last year's Christmas party at No 10 have been met with anger and heartache on social media.
People have been sharing how they spent 18 December 2020, from working long shifts in hospitals through to losing loved ones.
Posting on a BBC Facebook page, Louise Batten said she lost someone to Covid in January after people continued to meet up and break rules over Christmas and New Year.
"Ross was a home carer and had to carry on caring despite others meeting up," she wrote. "My son blames the government. Shame on them."
Rachel Rahman said stories where government officials broke the rules, including Dominic Cummings' trip to Barnard Castle, "stuck in my throat" as she had been unable to spend time with her terminally-ill mother because of lockdown restrictions.
And David Fish wrote: "So many people did the right thing last year, including not being with families. Obviously this lot have different rules to the rest of us."
But Doreen MacLening, who lost her husband this time last year to Covid, said she was not angry at the government and "neither would my husband be".
She added: "The virus killed him, not the government or anything they did or didn't do."