Pre-departure Covid-19 testing will be required for everyone travelling to the UK from 04:00 GMT on Friday morning.
People arriving by plane, train or boat, including UK nationals, will have to take a test up to 72 hours before leaving the country they are in.
All those arriving from places not on the UK travel corridor list must still self-isolate for ten days.
The plans were unveiled last week, though when and how they would be implemented was not confirmed.
There have been issues with testing availability and capacity, so some countries will initially be exempt, the government said.
For instance, the requirement will not apply to travellers from St Lucia, Barbados, Antigua and Barbuda for six days.
Travellers from Falkland Islands, Ascension Islands and St Helena are exempted permanently.
Hauliers are exempt to allow the free flow of freight, as are air, international rail and maritime crew.
The announcement comes after a further 529 deaths within 28 days of a positive test were reported in the UK on Monday. There were also 46,169 cases reported.
More than 32,000 people are currently in hospital with coronavirus, data shows.