India was surprised by the UK’s decision to impose a travel ban on the country despite surging infections and scientific concern over a new variant of the virus.
Britain on Monday added the subcontinent to its red list of travel ban countries after confirmed cases of the Indian variant in the UK rose to 103.
Health Secretary Matt Hancock said the “difficult but vital decision” would take effect from 4am London time on Friday.
Returning British citizens will have to pay to quarantine for 10 days in a government-approved hotel, while others will be refused entry.
The move came hours after Prime Minister Boris Johnson cancelled his planned trip to India next week, where he had hoped to boost ties between the nations as part of his post-Brexit trade vision.
Nalin Kohli, spokesman for Indian Prime Minister Narendra Modi’s Bharatiya Janata Party, said the travel ban came as a surprise, given the “lack of data” to suggest the Indian variant was more dangerous than previous strains.
"The UK is in its sovereign rights to do as it sees fit, but it will cause a lot of inconvenience to people who are travelling," he told the BBC Radio 4 Today programme on Tuesday.
“It came as a surprise news item so I don’t know if there's an explanation because there’s nothing in the public space.”
An estimated 900 people arrive in the UK each day from India, with five flights landing on Monday.
There is likely to be a rush of travellers booking flights to Britain before hotel quarantine measures take effect.
According to Skyscanner, there are no direct flights available between New Delhi and London Heathrow Airport on Tuesday or Wednesday.
Prof Sir Mark Walport, former chief scientific adviser for England, said the decision to place India on the red list was taken too late.
"These decisions are almost inevitably taken a bit too late in truth, but what's absolutely clear is that this variant is more transmissible in India," he said.
:You can see that it's becoming the dominant variant and the other concern about it is that it has a second change in the spike protein which may mean that it's able to be a bit more effective at escaping an immune response, either a natural one or vaccine-induced one, so there's good reasons for wanting to keep it out of the country if at all possible."
Education Secretary Gavin Williamson defended the government’s decision to wait until the end of the week for the changes to take effect, and said it was “standard practice to give a short window”.
“The government continuously reviews the data … sadly India is one of those countries that have to be added,” he told Sky News. “We have to remain vigilant against new variants – that’s why we have to place India on the red list.”
India is recording more than 200,000 new cases per day, and the health system is said to be crumbling under the weight of new infections, with Covid-19 patients housed in sleeper trains converted into clinics.
Britain’s main opposition Labour party said the government should have acted sooner.
“It’s not good enough to try and shut the door after the horse has bolted, by adding countries on to a red list when it is too late,” Shadow Home Secretary Nick Thomas-Symonds said.
Mr Johnson's scaled-down trip to India was due to take place next week but the tour has been scrapped.
"In the light of the current coronavirus situation, prime minister Boris Johnson will not be able to travel to India next week," a joint statement from the British and Indian government said.
"Instead, prime ministers [Narendra] Modi and Johnson will speak later this month to agree and launch their ambitious plans for the future partnership between the UK and India.
"They will remain in regular contact beyond this, and look forward to meeting in person later this year."
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The Delhi region announced a week-long lockdown on Monday. Fewer than 100 critical care beds were available in the city of New Delhi, with a population of more than 20 million people, Chief Minister Arvind Kejriwal said.
UK health authorities said the Indian strain was a “variant under investigation” but this could progress to a “variant of concern” if it is discovered to spread more easily or undermine vaccines.
Prof Danny Altmann from Imperial College London said that new variants should be treated with caution as vulnerable people could be “caught out” if the strain was able to evade immunity.
“We are still vulnerable, and some people in our population are still vulnerable – what I mean by that is the Indian variant, for example, certainly has a mutation like the ones that evade the best neutralising antibodies,” he said.
“If you have a population where at least half of us have had zero or one dose of vaccine, some won’t have made a very good response to the vaccine, because perhaps they are very old or obese or unwell.
“We still have a very large vulnerable population who can still be caught out by variants like this.”
Mr Johnson's visit to India was regarded as an exciting opportunity to upgrade UK-India relations after a series of false starts.
Walter Ladwig, a political scientist and South Asia expert at King’s College London, said there was an appetite on the Indian side for a transformation of bilateral links.
"We're now seeing very clear signals, very obvious signals from the Modi government that they are interested in upgrading relations with the UK," he told The National.
“They see the UK as being one among many states that can play an important role with respect to India and the Indo-Pacific.”
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