France, the Netherlands, Malta, Monaco and Turks & Caicos are among the countries that have been removed from the UK's safe list.
That means the thousands of UK citizens in those countries on holiday or business will have to isolate for 14 days.
Here's how that will affect holidaymakers and employers who have staff currently in those countries.
When will I have to self-isolate?
The new measure comes into force from 4am on Saturday August 15. Anyone returning after that date will be required to self-isolate for 14 days.
It applies to people returning to England, Scotland, Wales and Northern Ireland.
I'm in one of the countries on the quarantine list, what should I do?
The Foreign and Commonwealth Office (FCO) is not advising holidaymakers who are already in the listed countries to leave immediately.
People are instead being asked to follow local rules, return home as normal and check the travel advice pages on gov.uk for further information.
I've got a holiday booked to a country on the list, what should I do?
The FCO has advised British nationals against "all but essential travel" to the the countries on the quarantine list. Those who still decide to travel after August 15 will have to self-isolate for two weeks.
What will happen with my travel insurance?
The Association of British Insurers (ABI) said travel insurance will remain valid for people who are already in the quarantined countries until they return home.
However, those who travel to the listed countries after the FCO advice has changed would "likely" find their insurance invalid, the ABI said.
What about employers whose employees have to go into quarantine?
Foreign Secretary Dominic Raab previously said no worker following quarantine guidance should be penalised by employers, including by being put onto sick pay.
He said that if someone is following the law in relation to quarantine and self-isolating, "they can't have penalties taken against them".
