LONDON — New measures to combat the new omicron variant of coronavirus came into force in England Tuesday, with face coverings again compulsory in shops and on public transport.

From Tuesday morning, all travelers returning to the U.K. must also take a PCR test and self-isolate until they receive a negative result. Previously they had been able to take a lateral flow test and there was no requirement to isolate.

The reintroduction of mandatory face masks brings England closer in line with the rest of the U.K. — Scotland, Wales and Northern Ireland — which had not relaxed coronavirus restrictions as much as England had done since the summer.

About 14 cases of the omicron variant have so far been identified across the U.K. Prime Minister Boris Johnson said the new measures will “buy us time in the face” of the new variant.

Johnson’s government announced Monday it is expanding its booster vaccine program, with a booster dose to be offered to all those over 18 years old three months after people had their second jab.

Up until now, only people 40 years old or over and those deemed clinically vulnerable were eligible for the booster shot.

The change in advice means around 13 million more people will be eligible for the vaccine. So far, the U.K. has given around 17.8 million booster shots.

Johnson is expected to encourage more to take up the booster shots in a press conference later Tuesday.

Jenny Harries, who heads the U.K. Health Security Agency, said that while there was still uncertainty in understanding the omicron variant, officials hope that the expansion of the booster shot rollout will “to some extent counter the potential drop in vaccine effectiveness we might find with this variant.”

She also urged people to be cautious and reduce socializing over the festive season if possible.