All arrivals from overseas will have to take a Day 2 PCR test from tomorrow, Health Secretary Sajid Javid confirmed.
It means they are required to isolate until a negative result.
Any personal contact with a case of Omicron will require a spell of isolation even those who are fully-vaccinated.
The red list has now expanded to 10 African countries with cases of the Omicron variant.
ABTA called it a ‘huge blow’ for a travel industry still in the early stages of recovery.
"It’s vital that this decision is kept under careful review and restrictions are lifted promptly if it becomes clear there is not a risk to the UK vaccination programme," it said.
The Health Secretary said the measures ‘buy time’ as scientists scramble to learn more about Omicron.
"Now the Government has taken the decision to make PCR tests mandatory, it must take steps to properly regulate the marketplace and implement the Competition and Markets Authority recommendations so consumers can have confidence they are booking with a provider they can rely on," Which? Travel editor Rory Boland said.
Another new measure announced at the weekend by Boris Johnson is the return of masks in public.
Masks are mandatory in shops and on public transport.
It comes as a third case of Omicron was identified.
