Three types of the new Indian COVID variant are in the UK, new data shows.
Public Health England (PHE) has designated one of them – B.1.617.2 – as a variant of concern.
The other two Indian variants (B.1.617.1 and B.1.617.3) remain "variants under investigation".
In the past week, the total number of cases of the Indian variant of concern confirmed in Bolton has doubled, increasing from 345 registered between 6 to 12 May to 675 in the seven days between 29 April to 5 May.
The mayor of Greater Manchester, Andy Burnham, spoke of a "worrying" spike of cases among under 25s in the town and has asked for everyone over the age of 16 there to be offered a COVID-19 vaccine.
Prime Minister Boris Johnson has said he is "anxious" about the variant of concern, and a member of the Independent Sage group of experts has said the growth of the new variant leads her to conclude that next week’s planned relaxation of measures should be delayed.
But on Friday, vaccines minister Nadhim Zahawi insisted to Sky News that step three of England’s lockdown easing will still go ahead on Monday, despite the soaring cases of the Indian variant.
He said Monday’s plans would remain in place "because the vaccines are delivering" and "at the moment we have no evidence it [the variant] escapes vaccines".
The highest number of cases of the Indian variant of concern at the beginning of May were in Bolton, followed by Blackburn with Darwen, Leicester, Nottingham and Bedford, according to Sanger Institute data.
Bolton has the second-highest current infection rate in the country, only after Erewash.
The real number of cases of the Indian variant in these areas could be much higher. Sanger’s data shows only the number of samples of B.1.617.2 which have been found during genomic sequencing in laboratories.
There is also concern about the growth of the Indian variant in Blackburn with Darwen. Cases of the variant of concern were found in the Shear Brow and Corporation Park, Billinge and Beardwood, and Bastwell and Daisyfield areas of the town. People aged over 18 in those three wards were told they could book an appointment to get a Pfizer COVID vaccination.
Blackburn with Darwen has the third-highest infection rate in the country, and it has also seen the number of cases risen recently by 43%.
Across England, the number of cases of the Indian variant more than doubled in just one week, from 202 to 520 (week ending 5 May).
But data from Sanger still show the vast majority of the cases in the UK are the Kent variant which accelerated towards the end of last year.
The rising number of Indian variant cases has forced the government to consider bringing forward vaccine second doses in the North West.