LONDON: Britain will require all arriving passengers to isolate until they can show a negative PCR test against Covid-19 and is restoring a mandate to wear face masks in shops and public transport, Prime Minister Boris Johnson said Saturday.

"We now need to go further and implement a new testing regime," he told a hastily arranged news conference, hours after the government confirmed the first two cases of the new Omicron variant in Britain.

Currently, all Britons and foreigners entering the UK are required to take a PCR test on day two after their arrival.

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The new rules add the requirement for isolation pending a negative result, significantly toughening the regime, in a bid to curb the spread of the new strain.

"I very much hope that we will find that we continue to be in a strong position and we can lift these measures again," Johnson said, promising a review in three weeks, before Christmas.

"But right now this is a responsible course of action," he said, vowing to protect Britons' hopes for a more festive Christmas this year as he indicated no further nationwide lockdown is coming.

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Johnson added that the mandate for masks, controversially ditched by the government earlier this year, would return, without specifying when the new measures will come into force.