All British and Irish citizens and UK residents who arrive in England after being in a high-risk Covid country now have to quarantine in hotels.
The “red list” of 33 countries includes Portugal, Brazil and South Africa.
The new regulations, which aim to stop Covid variants entering the country, apply to arrivals who have been in one of those places in the past 10 days.
They will have to pre-book and pay £1,750 to spend 10 days quarantining in government-sanctioned hotels.
That covers the cost of the hotel, transport and testing.
Health Secretary Matt Hancock said the emergence of new variants meant the government “must go further” with its travel restrictions.
The government says it has struck deals with 16 hotels so far, providing 4,963 rooms for the new quarantine system, with a further 58,000 rooms currently on standby.
“The rules coming into force today will bolster the quarantine system and provide another layer of security against new variants at the border,” Mr Hancock said.
The health secretary added that the measures were “important to protect our vaccination programme”.