Mr Sunak, chancellor at the time, said he flew home from abroad and “fought very hard against the system” when ministers were allegedly “hours away” from announcing new restrictions.
The government eventually decided against new measures for England, a decision seen by ministers as vindicated when worst-case predictions did not come to pass. Some restrictions were introduced in Scotland, Wales and Northern Ireland.
The opposition had questioned Mr Sunak's whereabouts and accused him of being “missing in action” as concern over Omicron mounted, until he landed back in Britain from the US on December 17.
He told LBC radio on Thursday: “What I did in December was fly back from a government trip I was on overseas and I flew back to this country to stop us sleepwalking into a national lockdown, because we were hours away from a press conference that was going to lock this country down again.”
“And I came back and fought very hard against the system because I believed that would be the wrong thing for this country, with all the damage it would have done to businesses, to children’s education, to people’s lives.”
His account did not go unquestioned. Reports at the time suggested Mr Sunak's rival for the leadership, Foreign Secretary Liz Truss, had also opposed new restrictions.
But Mark Harper, a lockdown sceptic on the Tory back benches, said Mr Sunak had helped stop the cabinet “being bounced into a pointless Omicron lockdown … I know Rishi was on our side.”
Ministers announced limited coronavirus measures in early December, including compulsory face masks in some places and the partial use of vaccine passports.
Some countries brought back more stringent limits on social gatherings and public events as Omicron cases mounted, but Prime Minister Boris Johnson announced on December 27 there would be no new restrictions for the time being.
The partial measures were repealed in January once cases settled down, and the last remaining domestic restrictions were scrapped in February.
Mr Sunak is now battling to overhaul an early poll lead for Ms Truss in the seven-week battle to win over the Tory membership and be elected Britain's next prime minister.
Although criticised by some for prioritising the economy over public health during the pandemic, Mr Sunak is praised by allies for having overseen the £70 billion ($84bn) furlough scheme for workers.
His resignation as chancellor this month ignited a mass ministerial mutiny that culminated in Mr Johnson announcing his resignation.