The Indian Covid-19 strain has been declared a 'variant of concern' by Public Health England due to fears to could be more transmissible than the Kent mutation.
The Indian variant, known officially as B.1.617.2, could spread "at least" as fast as the strain found in Kent in late 2020, which eventually led to the Prime Minister putting England back into lockdown.
Figures show the cases of the Indian variant have grown from 202 to 502 in a week, almost half of which are related to travel.
But experts say there is currently "insufficient evidence" suggesting that the variant can make vaccines less effective.
"There is currently insufficient evidence to indicate that any of the variants recently detected in India cause more severe disease or render the vaccines currently deployed any less effective," PHE said in a statement, reported by Mirror Online.
The cases are spread across the country, with the majority in two areas - the North West, mainly in Bolton, and London, PHE said.
The decision to reclassify the variant is based on evidence which suggests it is at least as transmissible as a strain known as the Kent variant, PHE said.
The Prime Minister said today: "I think we've got to be very careful about that.
"We're doing a huge amount, obviously, to make sure that where we do find outbreaks of the Indian variant that we do surge testing, that we do door-to-door testing and isolate it.
"And also have very tough controls on transport and on people coming in."
