LONDON (Reuters) – Britain is tightening border controls to prevent new strains of COVID-19 coming into the country, suspending all the ‘travel corridor’ arrangements that had meant arrivals from some countries did not need to quarantine.

The change comes into force at 0400 GMT on Monday and means all passengers must have a recent negative coronavirus test and transfer immediately into isolation upon arrival. The isolation period lasts for 10 days, unless the passenger tests negative after five days.

Prime Minister Boris Johnson made the announcement during a news conference when he praised the country’s vaccination program, but he also warned: “What we don’t want to see is all that hard work undone by the arrival of a new variant that is vaccine busting.”

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

Britain has already felt the effects of mutations in the virus first hand. A strain first discovered in England has proved to be more transmissible and a major factor behind a spike in cases across the country.

(Reporting by Sarah Young, Writing by William James, editing by Elizabeth Piper)