Italy has been removed from from England, Scotland and Wales' Travel Corridor list following a rise in coronavirus cases, meaning anyone returning from there after 4am on Sunday, October 18, must quarantine for 14 days.
Anyone returning from Vatican City and San Marino after that date will also have to isolate.
But the Greek island of Crete is being added to the safe list of holiday destinations, meaning a two week quarantine will no longer be necessary for returnees after 4am on Sunday October 18.
The Department for Transport (DfT) said Italy was being removed because coronavirus data "indicated the weekly cases and test positivity are increasing".
Crete was added to the Travel Corridor list following a decrease in confirmed cases of coronavirus, the DfT added.
Scotland's Community Safety Minister Ash Denham warned Crete's addition to the list "is not a signal that it is safe to book foreign travel".
"All travel carries a risk and people should think long and hard before choosing to go abroad," she said.
Italy recorded its highest daily number of coronavirus cases on Wednesday, with 7,332.
On Thursday, the UK recorded 18,980 Covid cases.
The surge in Covid-19 infections in Italy has brought its seven-day rate of cases per 100,000 people to 64.
In contrast, the UK's case rate currently stands at 166.
Figures have been calculated by the PA news agency based on data collected by the European Centre for Disease Prevention and Control.
A rate of 20 is the threshold above which the government considers triggering quarantine conditions.
The Joint Biosecurity Centre said the risks to UK public health from travellers returning from Crete has decreased to an acceptable level and so it will be added back on to the list.
Transport Secretary Grant Shapps revealed the changes to England's Travel Corridor list in the usual way, with an announcement on Twitter.
The whole of the UK is now aligned with travel advice around all of the Greek islands.
In a statement the DfT warned the government would "rapidly" remove countries from the safe list if the risk to public health "becomes too high".