Washington, Dec 26 : A new order will require air passengers arriving in the US from the UK to ger a negative Covid-19 test, no more than 72 hours before departure, according to the US Centers for Disease Control and Prevention (CDC). Passengers are required to get a viral test within three days before their flight from the UK to the US departs, and provide written documentation of their laboratory test result to the airline, Xinhua news agency quoted the CDC as saying in a statement early Friday. Airlines must confirm the negative test result for all passengers before they board. If a passenger chooses not to take a test, the airline must deny boarding to the passenger, said the CDC. The new order will become effective on December 28. The initiative came as the public health authorities in the UK recently announced the discovery of two new variants of the novel coronavirus. Viruses constantly change through mutation, and preliminary analysis in the country suggests that this new variant may be up to 70 per cent more transmissible than the previously circulating ones, said the CDC. "The new order is consistent with the measures that have been taken so far to increase our ability to detect and contain Covid-19 proactively and aggressively," said the CDC. /IANS