Boris Johnson has been urged to act after an Indian COVID variant with potentially worrying mutations was found in the UK.
There are fears the variant, which features two mutations, could evade current vaccines, as well as immunity gained from previous infection.
Scientists said the appearance of the "double mutant" strain was a "disaster" and branded the prime minister "crazy" for still considering a trade trip to India later this month.
Experts have questioned why India has not yet been added to the red list of countries given the concern over the strain.
Public Health England reported that 73 cases of the B.1.617 variant have been confirmed in England as well as four cases in Scotland.
Specimens of the variant were first found in England in February, according to the Guardian.
It is understood that the cases detected in England are dispersed across different parts of the country and many are linked to international travel.
Officials have designated the strain a "variant under investigation" rather than the more serious "variant of concern", such as the Manaus (Brazil) or South African variants.
Dr Anthony Costello, a member of Independent Scientific Advisory Group for Emergencies, said: "This is a disaster. We have more than 50 flights from India daily. Seventy-seven cases of a really nasty India variant that affects younger people already here?
"Surge testing without isolation support. Contacts not being tested. This is a total shambles. We have learnt nothing after a year."
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Professor Christina Pagel, professor of operational research at University College London, had previously branded the the prime minister's planned trade trip to India "crazy".
But she said even though it had already been detected, it did "not make going okay".
She told The Independent: “The problem with this variant is that, not only does it have mutations that can evade antibodies, but they’ve found that it can also evade T-cells.
“And, as we open more and more, importing a strain becomes a lot more dangerous because it can spread a lot quicker before you notice it."
Downing Street confirmed today that the PM's trip will still go ahead.
A No 10 spokesman said: “As you would expect, safety is obviously important and is a priority for us on this trip, which is why we will make sure that all elements of the visit are COVID-secure.”
Deepti Gurdasani, senior lecturer in epidemiology at Queen Mary University of London urged the government to act, tweeting: "Not only do we are we continuing to see rises of the so-called South Africa variant, despite efforts to contain it, we also seem to have imported the double mutant from India – with *77* cases identified so far.
"When were these identified? And why isn't the govt acting?"
The B.1.617 variant was first identified in India and has since been found elsewhere.
Experts are worried about the variant as it carries two mutations that may make it more able to evade the body’s immune responses. It is also thought it may be able to infect the body more easily.
Paul Hunter, professor in medicine at the University of East Anglia, said the variant featured two “escape mutations” – E484Q and L452R – that “are causing people to be concerned”.
He added: “Basically, applying what we know about other human coronaviruses would suggest that this is going to be even less controlled by vaccine. But we don’t know that for certain at the moment.”
News that the strain has already been found in the UK comes as India reported a record daily increase of 217,353 COVID-19 infections in just 24 hours, with experts linking the sharp rise in cases with the mutant strain.
Worrying stories have emerged about unrelated patients being forced to share beds and bodies lying outside wards at one of India's largest COVID-only facilities as the country struggles to cope with the surge in cases.
India has recorded nearly 14.3 million COVID cases – second only to the US which has reported more than 31 million.
The prime minister has already scaled back his planned visit to Delhi, but is still planning to go at the end of April, with Downing Street insisting all elements of the trip will be "COVID-secure”.
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