The move, which was confirmed by health secretary Sajid Javid on 15 October, came into effect on Sunday (24 October), before the half-term holidays.
Javid said the switch was possible due to the "incredible" progress of the UK’s vaccination programme.
Travellers must send a picture of their lateral flow test as a minimum requirement to verify the result.
However, when the UK government launched the online portal for customers to purchase the day two tests on Friday (22 October), the service was frequently unavailable, while the cheapest option had already sold out.
Prices for the tests typically range from £20 to £40.
