Portugal has been removed from the UK's green list of countries which are safe to visit.
The government said it's partly down to the emergence of a "Nepal variant". The mutation has not been recognized yet as variant by WHO.
Announcing Portugal's move to the amber list, Transport Secretary Grant Shapps said there was a "Nepal mutation of the so-called Indian variant". He said it wasn't known whether it could be "vaccine defeating".
And speaking to the BBC, Communities Secretary Robert Jenrick said the Indian variant was spreading and there was "growing evidence of a further mutation- what's become known as the Nepal variant".
The government said it did not want to take risks before the planned final easing of England's restrictions. Being on the amber list means UK tourists should not visit Portugal and returnees must isolate for 10 days.
Is there a Nepal variant?
A mutated virus has been spotted in Nepal and elsewhere. But that doesn't mean a new variant of coronavirus has been found.
The WHO said it was "not aware of any new variant of SARS-CoV-2 being detected in Nepal", and the dominant variant there was the Delta or Indian variant.
A small number of cases of the Delta variant have been identified as having an extra mutation - called K417N.
The change is in the virus's distinctive spike protein and has been seen before, including in the so-called South African or Beta variant. Samples of the Delta variant with this extra change have been spotted about 90 times. Of these, 12 cases were spotted in Portugal, 36 in the UK, 12 in the US and four in India.
More up-to-date UK data from Public Health England suggests 43 cases have now been spotted through genetic analysis (a process called sequencing). The UK does a lot of genomic sequencing so is likely to find more cases than countries which do less.
The Wellcome Sanger Institute said it had been observed "once in Nepal (which does very little sequencing), and 14 times in Japan, of which 13 are samples from airport quarantine from travellers from Nepal".
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