The US became, on Christmas Day, the latest country to announce new travel restrictions because of the variant spreading in the UK and elsewhere.
Airline passengers from the UK will need negative tests within three days of their trip and to provide the results to the airline, the Centres for Disease Control and Prevention said in a statement.
The agency said the order will come into effect on Monday.
Airlines must deny boarding to any passenger who does not get a test, the CDC said.
The agency said because of travel restrictions in place since March, air travel to the US from the UK is already down by 90%.
Mr Johnson stressed there is “no evidence to suggest it is more lethal or causes more severe illness”, or that vaccines will be less effective against it.
The Associated Press contributed to this article