Everyone arriving in the UK from abroad must now self-isolate as the government order shutting all travel corridors comes into effect.
The tighter restrictions, intended to protect against the spread of coronavirus variants, were announced last week by Boris Johnson.
Travellers must also show proof of a negative COVID test taken in the three days before the start of their journey or face a potential £500 fine.
On arrival, they must isolate for 10 days – but can take another test after five days to end quarantine early.
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PM closes all travel corridors
The new rules will be backed by more spot checks to ensure people are remaining in isolation, but it’s still permitted to use public transport to get from an airport to home.
The isolation period also applies to UK nationals returning home, but travel from Ireland is exempt.
People arriving from many countries with low infection rates were previously listed on the government’s travel corridor list and did not need to isolate.
Exemptions to the policy – such as for business travel – are also suspended when the rules kicked in at 4am on Monday.
It comes amid concerns over two more COVID-19 variants identified in Brazil that have already prompted the government to ban all travel from South America and Portugal.
Scientists believe the vaccines being deployed in the UK will offer protection against the variants, but Chief Scientific Adviser Sir Patrick Vallance said it was a matter of “to what degree”.
The new regime will be in place until at least 15 February and the government has said it will continue to impose flight bans from countries where new variants are identified.
The UK airline industry, which has been one of the country’s biggest financial casualties of the pandemic, wants the travel corridors to return as soon as they can.
Tim Alderslade, chief executive of trade body Airlines UK, said last week that they were a “lifeline” for the travel industry when they were introduced in the summer.
He acknowledged that “things change” and it is the correct decision to remove them, but said restrictions should be eased “when it is safe to do so”.
Some airports might even be forced to temporarily close if the situation persists, according to the head of the Airport Operators Association.
“There is only so long they can run on fumes before having to close temporarily to preserve their business for the future,” said Karen Dee.