The UK has added seven new destinations to its travel green list, including six in Europe.
All passengers arriving in the UK from Canada, Denmark, Finland, Liechtenstein, Lithuania, Switzerland, and the Azores will no longer have to quarantine.
The Department for Transport said: "The risk that travel from these countries poses to public health in the UK is low."
It means there are just over 40 destinations that have quarantine-free travel with the UK for vaccinated and unvaccinated travellers.
Travellers from countries or territories on the green list do have to take a pre-departure COVID-19 test and a test within two days of their arrival in the UK.
They will also have to fill out a passenger locator form.
Thailand and Montenegro, meanwhile, will move to the UK's red list, meaning passengers from these areas will have "to isolate for 10 days in a managed quarantine facility and follow the necessary testing requirements," the Department for Transport said.
People who have been in a red list country or territory in the 10 days before they arrive in the UK must follow the same rules.
The changes will take effect from 4 am on Monday 30 August.
"The data for all countries will be kept under review and the government will not hesitate to take action should the data show that countries’ risk ratings have changed," the Department for Transport said in a statement, adding that the situation remains "fluid".