People who have been fully vaccinated in the US or European Union no longer have to isolate when arriving in the UK from an amber list country.
The rule change came into effect across the country at 04:00 BST on Monday.
The UK government has said the change, which was announced last week, will help to reunite family and friends whose loved ones live abroad.
Airline bosses have welcomed the move but are calling for more countries to be added to the UK’s green travel list.
Travellers will still need to take either a lateral flow or PCR test pre-departure and a PCR test on the second day after they arrive.
Under-18s will be exempt from isolation, and some will not have to test, depending on their age.
Since 19 July, people who have been double-jabbed in the UK have not had to isolate when arriving from amber list countries, but all other travellers are required to quarantine for 10 days when doing so.