Passengers walk past a sign in the arrivals area at Heathrow Airport in London. File / AP

A terminal at London's Heathrow Airport that was mothballed because of the coronavirus pandemic reopened on Tuesday to serve passengers arriving from high-risk countries - an action that critics said was long overdue.

Britain has barred travellers from a "red list” of 43 coronavirus hotspots including India, Brazil, Turkey and South Africa, fearing they could bring in worrisome virus variants.

Starting Tuesday, red list arrivals will pass through the airport’s Terminal 3, which was closed in April 2020 as international air travel plummeted, leaving Heathrow - previously Europe's busiest airport - largely empty.

UK citizens and residents who return from those countries face a mandatory 10-day supervised quarantine in a government-approved hotel.

Heathrow said in a statement that it had set up the new arrivals facility because "red list routes will likely be a feature of UK travel for the foreseeable future.”

Critics, however, have complained that red list passengers have been using the same airport arrivals hall as thousands of travellers from other destinations, though in separate lines, since hotel quarantines were introduced in February.

Some arriving passengers have described hours-long waits at Heathrow's passport control in crowded conditions.

The Public and Commercial Services Union, which represents border staff, said the decision had been made "at extremely short notice, meaning key social distancing procedures are not in place.”

"This is another poorly planned initiative that will be understaffed and rely on volunteers to do overtime, to avoid mounting queues,” the union said.

The UK has recorded almost 128,000 coronavirus deaths, the highest toll in Europe.

A mass vaccination campaign that started in December has brought new infections and deaths down sharply, but case numbers are once again rising as a more transmissible virus variant named Delta by the World Health Organisation (WHO), first identified in India, spreads across the UK.