Pre-departure Covid-19 testing will now be required for everyone travelling to the UK from 04:00 GMT on Monday.
The rules were meant to come into force early on Friday morning.
But Transport Secretary Grant Shapps said via Twitter that the government wanted to give people time to prepare.
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 government’s travel corridor list must still self-isolate for 10 days.
Problems with testing availability and capacity mean some countries will initially be exempt.
For instance, the requirement will not apply to travellers from St Lucia, Barbados, Antigua and Barbuda until 04:00 GMT on 21 January.
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.
Mr Shapps reminded travellers to fill out the Passenger Locator Form, used in track and trace, or face a fine.