British Prime Minister Boris Johnson announced Friday that travel corridors into the nation would be closed to fight the spread of COVID-19.

At a news briefing at his residence, Johnson said that effective Monday at 4 a.m. Britain time, all travelers into the country must show proof of a negative COVID-19 test taken within 72 hours and then self-quarantine for 10 days. They can take another test after five days and leave quarantine if that test is again negative.

Johnson said that the British government would be stepping up enforcement of the rules at the border and elsewhere in the country and that substantial fines would be imposed on those who refused or failed to comply with the measures. The border closures will be in effect until at least February 15.

The prime minister said these measures were being taken to prevent new strains of COVID-19 from entering the country, just as the government has begun to make progress with its vaccination program. "What we don't want to see is all that hard work undone by the arrival of a new variant that is vaccine busting," he said.

On Thursday, Britain banned arrivals from South America, Portugal and some other countries over fears about a strain of the virus detected in Brazil.

Friday, Britain reported 55,761 new confirmed COVID-19 cases, up from 48,682 the previous day. The prime minister warned that the National Health Service was facing "extraordinary pressures," having had the highest number of hospital admissions on a single day of the pandemic earlier this week.