He has repeatedly said he was assured the gathering complied with social distancing restrictions in place at the time and has ordered an internal investigation.
But the Sunday Mirror published a photograph of Johnson said to have been taken three days before that party, at a “virtual quiz” for his staff, in which four teams of six people took part to raise funds for charity.
Christmas lunches and parties that were “primarily social activity” were banned at the time, as Britain — one of the worst-hit countries by Covid-19 — battled a surge in cases of the Delta coronavirus strain.
Mixing of more than two people from two households indoors was banned, scuppering Christmas plans for millions after months of tight restrictions, including lockdown.
The left-leaning Sunday Mirror said Johnson took part for up to 15 minutes and asked questions as quizmaster in one round.
Education Secretary Nadhim Zahawi dismissed the latest report, telling Sky News television: “Many people would have had similar Zoom quiz nights around the country.”
But the main opposition Labour party said it showed Johnson presided over “a culture of disregard for the rules at the heart of government”, after a slew of claims that parties were held across Whitehall last Christmas.
An Opinium poll for The Observer put Labour nine points ahead of Johnson’s Conservatives, and suggested 57 percent of people think he should quit — up nine points from just two weeks ago.
Johnson’s popularity rating fell to -35 percent, down from a record low of -21 percent in the same period, the newspaper said.
The claims of double-standards have put Johnson’s position under threat at a crucial time.
MPs vote next week on the government’s latest proposed restrictions to curb the spread of the Omicron variant, with many Tories unhappy at plans to introduce mandatory vaccine passports.
Johnson also faces a by-election in a safe Tory seat vacated by an MP found to have illegally lobbied ministers on behalf of two companies that had him on their payroll.
A significant rebellion in the House of Commons and either a defeat or substantial loss of majority in Thursday’s by-election could see calls for Johnson to go reach fever pitch.
One commentator said the by-election was effectively “a referendum” on his premiership.