Cases of Covid-19 caused by the Delta variant, which first originated in India, have risen by 5,472 since last week to 12,431.
Public Health England (PHE) suggests the variant has now overtaken the Alpha variant, which first originated in Kent, as the most dominant variant in the UK.
Last week, health secretary Matt Hancock said that up to three-quarters of new coronavirus cases in the UK were the Delta variant, when 6,959 cases were confirmed.
What is the Delta variant?
There are a few variants that originated in India circulating, but one is causing more worry than others.
The strain – B1617.2 (or the Delta variant) – is one of three related variants that have been detected in the UK. The others are B1617.1 and B1617.3.
There were originally four variants of concern (VOCs) in England – the so-called Kent, South Africa, Brazil and Bristol variants.
All three variants from India were originally designated as variants under investigation, however Public Health England (PHE) confirmed B1617.2 had become the fifth variant of concern as of May 7 due to rising cases.
Since then, cases have soared on a weekly basis, with scientists suggesting this new variant is more transmissible than the Alpha variant.
Early evidence also suggests there may be an increased risk of hospitalisation, although more data is needed to confirm this.
PHE said 278 people with the Delta variant attended A&E this week, resulting in 94 people being admitted to hospital overnight. Last week, 201 people attended A&E, with 43 admissions. The majority of these had not been vaccinated.
Which areas are worst affected?
Bolton remains one of the most affected areas, where cases have risen by 795 to 2,149. Blackburn with Darwen has also seen 368 new cases, bringing it to 724 in total.
There are encouraging signs that the transmission rate in Bolton has begun to fall, PHE said, and that the actions taken by residents and local authority teams have been successful in reducing spread.
Other areas in England with more than 100 confirmed cases of the variant, as of last week, included: Leicester, Sefton, Nottingham, Wigan, Central Bedfordshire, Manchester and Hillingdon.
The health body also published a breakdown of outbreaks and clusters of variants in schools and other settings.
The latest data suggests there have been 97 confirmed Covid-19 outbreaks in primary and secondary schools that have had at least one variant case linked to them over the most recent four-week period.
This represents around one in 250 schools.
What does it mean for lockdown easing?
As it stands, the lifting of restrictions on June 21 hangs in the balance.
PHE experts urged the public to “remain cautious” as the country approaches the next stage of the roadmap.
Variant cases are on the increase in several areas and it is absolutely crucial that everyone plays their part in preventing their spread, PHE said.
Dr Jenny Harries, chief executive of the UK Health Security Agency, said: “With this variant now dominant across the UK, it remains vital that we continue to exercise caution particularly while we learn more about transmission and health impacts.
“The way to tackle variants is to use the same measures to reduce the risk of transmission of Covid-19 we have used before. Work from home where you can, and practise hands, face, space, fresh air at all times.”
She urged those who are eligible to get vaccinated as soon as possible.