Proof of a negative COVID-19 test for England arrivals will apply now from Monday, not Friday as the government originally suggested.
Nationals trying to get home could still be left out of pocket or risk being stranded, if they can't get tested in time for travel.
Scotland and Wales' testing requirements for arrivals will also begin on 18 January at 4am, but there is currently no requirement to get a test if you're travelling to Northern Ireland.
Almost everybody travelling to England, Wales or Scotland will be required to get a reliable Covid-19 test. This could be the standard PCR test (used by the NHS and many other countries) or a cheaper, faster LAMP or antigen test. However, it's the responsibility of the traveller to check with their test provider that the test meets minimum requirements (see below).
Passengers who test positive will need to self-isolate and may then face an expensive and difficult journey home, with many flights to the UK having been suspended from next week.
Even those who don't have Covid could have problems in many parts of the world, because of the difficulty in getting a test at short notice.
The government has allowed holidaymakers in Antigua, Barbados and St Lucia more time to be able to fly without a test. They have until 21 January due to lack of testing infrastructure. Travellers from Falkland Islands, Ascension Islands and St Helena do not need a test but people may struggle in many parts of the world.
Travellers will need a Covid-19 test to fly to England or Scotland no more than 72 hours before departing. But if the result is not returned before a passenger's scheduled departure, no insurer Which? contacted would cover passengers who need to purchase new return journeys.
AllClear, Allianz Assistance UK and AXA UK have confirmed that if you can't get a test result returned within the 72-hour window required, you won't be covered for the cost of a new flight under their policies.
If passengers don't comply with testing requirements, they face a £500 fine in England and £480 fine in Scotland. Excluded will be the likes of hauliers, children under the age of 11, crews, those who are travelling for urgent medical reasons and those travelling from countries without the infrastructure available to deliver tests. See the government website for a full list of exemptions.
If you are currently abroad, Which? advises you locate potential private clinics now to avoid a last-minute dash.
To be confident that the test meets minimum requirements travellers might want to choose PCR tests rather than antigen, as these are more reliable. However, they take longer to return the results and are usually more expensive. If you do choose an antigen or other test you need to check with the test provider that it meets the government's minimum standard of '97% specificity, 80% sensitivity.' .
No, you cannot get a test on arrival, you need to get it before you leave the country you're travelling from.
If you can't locate a test until after your departure date, or a test does not get returned to you in time, you'll be required to book a new flight. Meeting entry requirements is the responsibility of the passenger, even when these change at the last minute.
You may be able to change your time or date of travel if you've booked a flexible ticket. If your ticket is a standard fare, contact the provider to ask if it will let you move your ticket free of charge or for a small fee.
You can't fly. Your insurers may cover you for accommodation, travel and health costs if catching COVID-19 abroad was covered in your policy.
AllClear, Allianz Assistance UK and AXA have confirmed this would be covered under their insurance, but you should check details with your own insurer.
Yes, you'll need to if you're travelling from somewhere not on the governments travel corridor list, even if you test negative.
You can reduce isolation time if you participate in the , where you will pay to be tested on the fifth day after landing in the UK. You'll only need to self-isolate for five days if you test negative.
If you're searching for a clinic yourself, it's important to note Italy's regions (such as Sicilia, Lazio, Veneto) manage their own healthcare services. For example, information on testing in the Lazio region, around Rome, can be found .
You'll need to locate a drive-in or authorised medical centre to gain a PCR test in Italy. Costs can vary depending on region - in Lazio, the test is capped at 60 euros.
Many airports provide antigen tests, but we don't yet know if an antigen test will be accepted by the UK government.
If you don't speak Italian, it's probably easier to ask a local pharmacy or hotel staff to help you.
The government told Which? that passengers should search for a private testing clinic in the area they're currently travelling in. It told us it will give further advice to travellers next week.
It's important to check how quick the turnaround time is, as you'll need the result ready for when you travel, but you can't take it earlier than 72 hours before your flight's departure time.