Those arriving by plane, ferry or train - including UK nationals - must give an address where they will self-isolate. Rule breakers will be fined. Home Secretary Priti Patel has said the laws were designed "to prevent a second wave" of coronavirus. But the boss of Ryanair has said the rules are a "political stunt" and are not a quarantine. Michael O'Leary told the BBC: "You could be in Sainsbury's, you could be on the beach, you could be on the golf course in the unlikely event the Home Office calls you - all they will have is a mobile number."
He claimed even the Home Office acknowledged the rules were unenforceable. Some industries have warned they will be severely impacted by the rules, and Mr O'Leary warned of devastation. Despite criticism from businesses, Ms Patel has said the measures are "proportionate" and being implemented "at the right time".
What are the new rules?
- People arriving in the UK should drive their own car to their destination, where possible, and once there they must not use public transport or taxis.
- Arrivals must not go to work, school, or public areas, or have visitors - except for essential support. They are also not allowed to go out to buy food, or other essentials, where they can rely on others.
- Those arriving in England, Wales and Northern Ireland could face a fine of £1,000 if they fail to self-isolate for the full 14 days, while they face a £480 fine in Scotland. The maximum fine for repeat offenders in Scotland is £5,000.
Almost all travellers have to fill in a "public health passenger locator" form on arrival. Failure to do so could lead to a penalty of £100, or travellers may be refused entry. If they are unable to provide an address, the government will arrange accommodation at the traveller's expense. It says there will also be checks to see whether the rules are being followed.
Anyone arriving from the Republic of Ireland, the Channel Islands or the Isle of Man does not have to complete a form or enter quarantine. There are also exemptions for workers in some industries such as road haulage and medical professionals who are providing essential care.