London's pubs and restaurants are to close this week as authorities confirmed a new mutation of COVID-19 was spreading in southeast England.
Health Secretary Matt Hancock said that the "new variant" had been identified by scientists and could be associated with a faster spread of coronavirus.
"Initial analysis suggests this variant is growing faster than the existing variants," he told parliament on Monday.
"We've currently identified over a thousand cases with this variant, predominantly in the south of England, although cases have been identified in over 60 different local authority areas, and numbers are increasing rapidly.
"Similar variants have been identified in other countries over the last few months."
Hancock said the UK had notified the World Health Organisation about the new variant.
Scientists believed it was "highly unlikely" that the COVID-19 vaccine would prove ineffective against the new variant, he added.
London closures announced
It came as officials confirmed London would move from Wednesday into Tier 3, the highest level of coronavirus restrictions implemented in England.
The suburbs of Essex and Hertfordshire, to the north and east of the capital, will also move up to the high tier at the same time.
The measures will come into force one minute after midnight on Wednesday morning.
Under the measures, pubs and restaurants can no longer accept sitting customers and must close entirely if they do not offer a takeaway service.
Tier 3 rules also mean professional sporting fixtures cannot admit spectators.
Top-flight London football teams including Arsenal, Chelsea and Tottenham Hotspur will have to play fixtures behind closed doors.
Wedding ceremonies are permitted for up to 15 people, but receptions may not be held.
London was previously in the less restrictive Tier 2.