Passengers to England and Scotland by air, sea and rail who arrive after 4am on Monday 18 January will be required to present a negative coronavirus test certificate before they are allowed to travel. They must pay for the test.
But what kind of test is needed? And are there any exceptions?
These are the essential questions and answers.
What is happening – and why?
The UK government is seeking to reduce the chances that coronavirus infections are brought in by incoming travellers.
The transport minister, Robert Courts, said: “With the addition of pre-departure testing requirements, our already robust system to protect against imported cases of coronavirus is further strengthened."
So pre-travel tests will be demanded from passengers before they board flights, ferries or trains to the UK.
Most people hoping to travel to England and Scotland will be required to provide proof of a negative test for Covid-19.
Under-11s, lorry drivers and crew for aircraft, ships and trains are exempt.
The measure was originally supposed to take effect at 4am on Friday 15 January, but the introduction has been postponed to the early hours of Monday 18 January.
A spokesperson for the Scottish government said: “Scotland was also obliged to delay implementation as we need sight of their final regulations in order to properly draft and approve the relevant Scottish regulations.”
It will apply to arrivals from all foreign countries except the republic of Ireland.
Wales and Northern Ireland have no significant international arrivals due to the lockdown rules banning leisure travel. But nationals of these countries who board flights to England or Scotland will need to comply.
How will it be enforced?
Airlines, ferry firms and rail operators will be required to ensure that travellers meet the requirement. They face a £2,000 fine for each passenger they let through without correct certification.
Anyone who gets through to the UK border without a test faces a fine of £500 (£480 in Scotland).
The UK Border Force is expected to carry out spot checks on travellers to monitor compliance.
What sort of test will be required?
Travellers who are planning to arrive from 4am on Monday 18 January are told they are solely responsible for finding a test provider that has “performance standards of ≥97% specificity, ≥80% sensitivity at viral loads above 100,000 copies/ml”.
The government says this could include polymerase chain reaction (PCR), loop-mediated isothermal amplification (Lamp) or lateral flow test. But the government says: “It is your responsibility to ensure the test meets the minimum standards for sensitivity, specificity and viral load details so you must check with your test provider that it meets those standards.
The original test result certificate must be on paper, or included as an attachment to an email or a text message on a mobile device. It must be in English, French or Spanish and include the contact details name of the test provider, the name of the test device and the date the test sample was collected or received.
“If the test result does not include this information you may not be able to board, and may not be able to travel to England,” the government says.
“If you arrive without a test result that includes this information, you will be committing a criminal offence and could receive a £500 fine.”
I’m in a foreign country. How am I supposed to know which tests qualify?
The government makes it clear that is your problem.
However, British Airways says it will accept a wide range of tests: "Unless exempt, we require all customers on flights to the UK arriving from 4am on Monday 18 January to provide evidence of a negative Lamp, PCR or antigen Covid-19 test taken within 72 hours of their departure.
“Customers who are unable to travel on their intended departure date will be able to move their flights to a later date using our ‘Book with Confidence’ policy.”
Is it feasible to expect enough tests to be available?
At present, very few British travellers are abroad, and almost no one is travelling to the UK. Therefore it is hoped that most locations will be able to meet the demand.
But there will be some places – such as the Caribbean islands – where the prospect of organising tests for hundreds of people all leaving on the same flight may be beyond the resources of the local health providers.
Travellers in Antigua, Barbados and St Lucia get an extra six days – until 4am on Thursday 21 January.
The government has given the Falklands, Ascension Island and St Helena permanent exemption.
What if I can’t find a test in time?
The government has made it clear that is your problem. You will not be allowed to travel.
One exception: travellers to England who are connecting through a hub such as Dubai or Amsterdam and who are unable to get a test at the transit point will be allowed to board their plane.
But the government warns that on arrival they could be hit with a £500 penalty, saying: "You should not rely on being able to get a test in a country that you will transit through as part of your journey to England.
"It is possible that local or entry restrictions will mean you are not able to get a test.
"If you don’t have a test result because you were unable to get one in a country you transited through, and you are not permitted to enter the transit country, you will be allowed to board your transport to England.
“But you could be fined £500 on arrival in England for not having a valid test result.”
Why can’t I take a test on arrival instead?
Many countries demand travellers undergo tests on arrival, either in addition to a pre-departure test or on its own. But the UK government insists that testing on arrival is ineffective.
What happens if I test positive?
You will be expected to self-isolate in accordance with local laws at your own expense until you are deemed well enough to travel.
What if I miss my flight home due to late results?
You will have to arrange to travel on a later flight (it is likely that airlines will be flexible with rebooking) and pay for accommodation until you can return home.
Will I need another test at the UK border?
No. The government has long insisted that tests immediately on arrival in the UK are pointless.
Must I still self-isolate?
Yes, if you are arriving from all but the dwindling number of “quarantine exempt” countries on the travel corridors list you will be expected to self-isolate for 10 days. In England, the traveller can end quarantine after five days with another negative coronavirus test.
I am only going abroad for two days
Bizarre as it may seem, you can take a test in the UK just before departing, rather than using any of your precious time abroad getting tested. This appears to go against the principle of a scheme designed to prevent infections being brought in from overseas.
Any test must be privately obtained. NHS tests should not be used and will not qualify.
Any exemption if I have had Covid or a vaccine?
No. People who have recovered from coronavirus, and those who have been vaccinated, are also expected to be tested and to self-isolate.
I have a holiday booked but I can’t afford this. Can I get my money back?
You will not be able to claim a refund. All travel contracts assume that the passenger will meet whatever governmental requirements are in place at the time of the trip.
What will this do to holiday and flight bookings?
It will prove extremely difficult to sell many holidays when people believe they could end up paying for tests both outbound and inbound, which in many cases could double the cost of the trip.
Michael O’Leary, chief executive of Ryanair, said: “This measure will destroy any confidence in forward bookings for air travel and for ferries, and it will essentially close off the UK.
“The real solution to this is vaccination, not artificial travel restrictions.”
He said that Ryanair has cancelled almost all UK flights in February, and called for the government to set an “end date” for the rules.
Tim Alderslade, chief executive of Airlines UK, said: "It is true that much of the sector has been lobbying for pre-departure testing – but this was always predicated on the government removing or reducing the quarantine period at the same time. Now we have both quarantine and pre-departure testing.
“That’s why this new policy needs to be a time-limited, emergency measure only, in place not a second longer than necessary, and with a proper review mechanism once lockdown comes to an end. We cannot afford for this to be baked in over the whole summer.”