After a two-hour cabinet meeting on Monday (20 December), the prime minister said the government was reviewing data on the new Omicron variant of Covid-19 on an "hour by hour" basis, and urged the public to exercise caution.

"Unfortunately, I must say to people, we will have to reserve the possibility of taking further action to protect the public and public health, and protect our NHS and we won’t hesitate to take that action," Johnson told the BBC. 

"But in the meantime, what I would say to everybody is please exercise caution when you go about your lives. Please think about the guidance and protecting yourself and your loved ones and please get a booster, please get a vaccination, it could not be more urgent.

"There are still millions of people in this country who haven’t got a vaccination let alone a booster."

Daily Mirror political editor Pippa Crerar tweeted on Monday that measures being looked at by government could include a return to "Step 2" restrictions.

Step 2 restrictions were last in effect in April, and would specifically ban overseas leisure travel. Downing Street is yet to give a formal indication of what restrictions are under consideration.

"We are looking at all kinds of things to keep Omicron under control and we will rule nothing out," said Johnson.