CASES of Covid caused by the Indian and South African variants are on the rise, the Health Secretary has said.
Not all cases have been in people who have travelled abroad, suggesting there is already community transmission of the strains.
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Matt Hancock told MPs that cases of the Indian variant have gone up to 103 from 77 on April 7, and from Friday India will need to go on the travel "red list".
He said: “We’ve recently seen a new variant first identified in India. We’ve now detected 103 cases of this variant, of which again the vast majority have links to international travel and have been picked up by our testing at the border.”
Labour’s shadow health secretary Jonathan Ashworth said scientists at COG UK - which monitor variants - had found 135 cases of the Indian variant.
He said it made the strain "the fastest growing variant in the UK in the last three weeks".
Professor Paul Hunter, an epidemiologist at the University of East Anglia, has also claimed public data says there are 160 cases already, but it could be closer to 400.
But surge testing has not been deployed to squash further spread.
The variant woud need to be classified as "of concern" by the Government in order for surge testing to be rolled out, but is still only considered as "under investigation".
Mr Hancock said: “(Mr Ashworth) is right to ask about surge testing to make sure that we limit the spread as much as is possible of the variant first found in India – and we will be doing that, I can confirm.”
Samples of the Indian variant have been analysed to see if it does have “concerning characteristics” as feared.
Mr Hancock did not reveal the findings of these early studies and if they meant the Indian variant had greater transmissibility or was resistant to treatments and vaccines.
He said: “After studying the data, and on a precautionary basis, we’ve made the difficult but vital decision to add India to the red list.”
It means anyone who is not a UK or Irish resident or a British citizen cannot enter the UK if they’ve been in India in the previous 10 days.
“UK and Irish residents and British citizens who have been in India in the past 10 days before their arrival will need to complete hotel quarantine for 10 days from the time of arrival," Mr Hancock said.
“These rules will come into force at 4am on Friday.
“India is a country I know well and love. Between our two countries we have ties of friendship and family. I understand the impact of this decision but I hope the House will concur that we must act.”
India, home to 1.4 billion people, is seeing record high levels Covid cases, with 200,000 infections recorded in a single day.
Mr Hancock also said that cases of the South African variant had gone up to 557 after a cluster of cases was discovered in south London last week.
Public Health England data says confirmed cases are at 600.
The Health Sec said two thirds of cases were "related" to international travel.
"However, we have seen small amounts of community transmission too," Mr Hancock revealed.
"As soon as these cases were discovered we acted quickly to put in enhanced testing, tracing and sequencing in Lambeth and Wandsworth."
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The largest testing operation to date has invited people in the London boroughs of Lambeth, Wandsworth, and the Rotherhithe ward of Southwark.
Barnet Council has also urged residents in Finchley N3 to get a test after a singular case was discovered there.
Mr Hancock said today: "I'd urge everyone who lives in these areas, whether you have symptoms or not, to get tested regularly and help us keep this variant under control."