Every traveller entering the UK now needs to take a PCR test within 48 hours of arrival.
It follows concerns about the newly identified Omicron variant of Covid, which has also led to quarantine being reintroduced for travellers from some African countries.
What is the new rule for PCR tests?
If you are entering the UK from a foreign country (apart from Ireland), you have two days to take a PCR test - a lateral flow test will no longer be accepted.
The PCR test must be booked before you travel, and bought privately from a government-approved list of providers.
While you are waiting for a result, you must self-isolate - whether or not you have been vaccinated. You can stop self-isolating if your test is negative.
Why has this change been introduced?
There are concerns that a new variant, known as Omicron - which was identified in southern Africa - may not respond as well to vaccines.
Health Secretary Sajid Javid said it could be the most infectious variant so far.
New heavily mutated Covid variant raises concern
What are the rules in popular destinations?
Which countries are on the red list now?
Ten southern African countries have been added to the UK's travel red list, because of fears about Omicron.
These are South Africa, Namibia, Zimbabwe, Botswana, Lesotho, Eswatini Angola, Namibia, Mozambique and Zambia.
The only people allowed to enter the UK from these countries are UK or Irish nations, or UK residents.
They will have to pay for and self-isolate in a pre-booked government-approved hotel for 10 days.
10 days (11-nights) for one adult - £2,285
Additional adult (or child over 11) - £1,430
Children aged five to 11 - £325
What other rules are there for travelling to the UK?
All travellers coming to the UK must fill in a passenger locator form - even if they are just passing through - which they need to fill in 48 hours or less before their journey.
Before travelling to the UK
Fully vaccinated passengers travelling to the UK from many other countries no longer have to take a Covid test before setting off.
This applies to people vaccinated in the UK, the EU, the US and dozens of other recognised countries - including Brazil, Hong Kong, India, Pakistan and Turkey. You must be able to prove you are fully vaccinated.
The changes also apply to under-18s resident in one of these countries, whether or not they are vaccinated.
What if I'm unvaccinated?
If you aren't fully vaccinated - and you're over 18 - you need to:
take a PCR or lateral flow/antigen test (not a free NHS test) in the three days before you travel to the UK
take a test on or before day two and on or after day eight after your return.
self-isolate at home for 10 days after arrival in the UK
Test to Release will remain an option for unvaccinated passengers to England who want to shorten their quarantine.
What are the rules in the US?
Vaccinated people who have had a negative test taken no more than three days before travelling can enter the US without quarantining.
Another test, three to five days after arriving in the US, is recommended.
Vaccine certificates - including the NHS Covid Pass and equivalents from Scotland, Wales and Northern Ireland - are accepted.
Children under 18 do not need to be vaccinated, but should also take a test after arriving.
Masks are mandatory on planes and in airports, and travellers must follow state and local rules at their destination.
What's happening in Europe?
Some European countries are tightening rules.
In France, from 15 December, over-65s must have had a booster jab to travel or visit restaurants and museums. It is not clear whether this will apply to UK visitors.
People in England and Wales can now show they have had a third dose using the NHS Covid Pass.
Rules can change quickly, not only for Europe, but for all countries. You should keep checking the UK Foreign Office rules for your destination for the latest updates.