Health chiefs today bumped up the threat from the even more transmissible Delta off-shoot amid fears it may have already infected 150,000 Britons.
The UK Health Security Agency (UKHSA), which took over from the now-defunct Public Health England, have labelled AY.4.2 a 'variant under investigation'.
This category is reserved for variants which are spreading in the UK that may be more transmissible or better able to evade vaccines than other mutant strains.
Other variants in this category include Mu, the most recently named variant, and Kappa which is a relative of Delta.
It is just one step below 'variant of concern', the label given to Alpha and Delta, which is reserved for mutant strains that could change the course of the pandemic.
Scientists said the threat level was escalated because of rising case numbers and 'early evidence' that it is more transmissible than Delta.
Some estimates suggest it is up to 15 per cent more likely to spread, although this is yet to be confirmed.
Health chiefs said 15,120 cases have been spotted to date, although this may represent just 10 per cent of the total because labs are only swabbing a fraction of Covid cases for variants. This suggests the real figure may be closer to 150,000.
The proportion of cases it makes up has more than doubled over the last month, with AY.4.2 now behind almost one in ten infections in the country. The hotspot is in Adur, West Sussex.
There have been suggestions that the variant may be elevated to 'Variant under Investigation'. If this is the case the World Health Organization is likely to give it the name 'Nu', which is the next letter in the Greek alphabet
AY.4.2 was first spotted in the UK in late June in London and the South East before spreading to the rest of the country. Only two dozen areas of England are yet to record a single infection.
It carries just two mutations — Y145H and A222V — which have previously been spotted on other mutant
