New variants of the coronavirus can "come out of a blue sky", Professor Chris Whitty has warned.
England’s chief medical officer issued the note of caution at a Downing Street news conference, following an uptick in cases of a variant of concern first found in India.
Prof Whitty also mentioned the strain that was first identified in Kent.
Data presented at the briefing showed that 520 cases of the worrying COVID-19 variant first detected in India were found in the UK up to Wednesday 5 May.
This is a rise of 318 cases on the week before.
India has suffered one of the worst outbreaks of the pandemic, with hospitals running out of oxygen and makeshift funeral pyres set up to burn victims’ bodies.
According to Johns Hopkins University, the country has had 22,662,575 recorded cases of the virus and 246,116 deaths.
Some 168,304,868 vaccine doses have administered, with 34,450,192 people fully inoculated – equivalent to 2.52% of the population.
Prof Whitty said: "What we know with all the variants is that things can come out of a blue sky – you’re not expecting it and then something happens – that happened with the B.1.1.7 (the variant first identified in Kent), that has happened to India with this variant as well.
"At this point in time, our view is that this is a highly transmissible variant, at least as transmissible as the B.1.1.7 variant – it is possible it is more transmissible but we’ll have to see."
He added: "Most of the (variants) are currently relatively stable and not increasing at a great rate, the one that is slightly concerning in terms of increasing as a proportion is the variant which has been described from India.
"In certain parts of the UK, and that does appear to be increasing – but from very low levels – over the last two weeks."
Prof Whitty said there were three strains identified in India – but the one officials are "worried" about in the UK is called B.1.617.2 which is a "variant of concern".
The scientist said the way to prevent new variants from arising was to "get on top of" the pandemic globally.
The World Health Organisation has said there is "shocking disparity" between vaccination rates when comparing wealthy nations to poor countries.