Rishi Sunak says he flew back from a government trip last December to stop the country going into a new COVID lockdown.
The former chancellor and Tory leadership candidate told LBC that the UK was "hours away from a press conference" announcing the measures due to the spread of the Omicron variant.
But he said he "fought very hard against the system" to stop the lockdown being introduced.
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As 2021 came to a close, Omicron became the dominant variant in the country and cases rose quickly, with Boris Johnson advising people to exercise "caution" around the build up to Christmas and health officials urging the public to limit their contacts.
Rumours were abound that the government would bring in new restrictions as companies - especially in the hospitality sector - called for more support as people cancelled events.
But on 16 December, Mr Johnson said there was no order for people to cancel gatherings and ruled out any lockdown "by stealth".
On the same day, the Treasury confirmed Mr Sunak would be cutting a trip short - reportedly by one day - to return from the US to hold talks with business leaders over the impact of the variant.
It followed questions around his whereabouts as cases rose, with Labour telling him to "leave Hollywood and come back to reality".
During an interview with Andrew Marr, Mr Sunak said: "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 because of Omicron."
He added: "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."
Mr Sunak made the remarks a day after reaching the final two in the Conservative leadership contest, facing Liz Truss on the ballot paper.
The foreign secretary is currently leading the race, according to a poll of Tory Party members, who will decide their next leader and the incoming prime minister over the next six weeks.
Mr Sunak said he was glad he "won" the argument last year, and said it should give voters confidence he was "prepared to push hard and fight for the things that I believe in even when that's difficult".