The US has extended its ban on European travel to include the UK and Ireland.
The UK had previously been excluded from a transatlantic travel ban from 26 European countries, announced last week by Trump to battle the spread of coronavirus.
The travel restrictions for Schengen-zone countries started on Saturday for 30 days, cutting into the Easter weekend.
Vice-president Mike Pence announced the restrictions will begin at midnight Monday night EST (0400 GMT on Tuesday).
He said the decision was taken on the advice of health experts, and added that Americans and legal residents would be able to return to the US. They will be “funnelled” through selected airports.
President Donald Trump has also announced during a press conference that he has taken the coronavirus test and is awaiting results within the next 48 hours.
Tui said in response: “We are aware that the United States authorities have taken the decision not to permit entry to anyone travelling from the UK to the USA from 16 March 2020.
“Customers due to travel to the USA between 16th and 18th March will have their holiday automatically cancelled and refunded. There is no need to call us.
“Customers due to travel between 19th and 23rd March we will call you to rearrange your holiday or offer a full-refund.
“Customers due to travel after 24th March please bear with us as we prioritise customers due to travel before 24th March. We would like to thank our customers for their understanding.”