Fully vaccinated travellers from the US and EU can now arrive in the UK without having to isolate.
The new rules came into effect at 4am on Monday but they have not yet been met by a reciprocal easing of restrictions for travellers from the UK.
The new rule applies to England, Scotland, Wales and Northern Ireland.
Transport Secretary Grant Shapps said: “We’re helping reunite people living in the US and European countries with their family and friends in the UK.”
Arrivals from France are the exception, with isolation still required regardless of vaccination status.
Meanwhile, there are fears that the government is considering creating an amber watchlist, which would include countries at risk of moving to the red list at short notice.
Arrivals from red list countries must spend 10 days in hotel quarantine at a cost of £1,750.
There is already a green watchlist, which includes countries on the green list that are at risk of being moved down to amber.
According to The Daily Telegraph, Mr Shapps is against the idea and is instead pushing for travel to be opened up further.
Ministers will meet on Thursday to decide on the travel rules that will be in place for most of August.
The Department for Transport said it would not comment on “speculation” when asked about a possible amber watchlist.