The Department for Transport on Thursday updated the UK travel list, which details what precautions are needed for people to travel between the UK and other countries.
Thailand and Montenegro were added to the red list. This means travellers arriving from the two destinations will need to check in to a quarantine facility for ten days on arrival.
They will also undergo regular testing.
Meanwhile seven countries - Canada, Denmark, Finland, Liechtenstein, Lithuania, Switzerland, and the Azores - have moved on to the green list, meaning they are among the easiest countries to travel to from the UK.
The Azores are a Portuguese archipelago in the mid-Atlantic.
The changes will come in to force at 4am on August 30.
A statement from the department said: "The high rates combined with lower levels of published genomic surveillance in Thailand and Montenegro than other countries, mean that an outbreak of a new variant or existing variants of concern (VOC) or variants under investigation (VUI) cannot be easily identified before it is imported and seeded across the UK."
People travelling from green list countries do not have to quarantine on return to the UK, regardless of vaccination status, but still need to take pre-departure and day two tests - and complete a passenger locator form.
People arriving from red list destinations have to isolate at a managed quarantine facility for 10 days, and be tested as necessary.
Bulgaria, Croatia, Austria and Australia are among the current green list countries.
South Africa, Argentina and Brazil are on the red list.