The number of UK cases of the Indian COVID variant, also known as Delta, has risen by more than 5,000 since last week.
Cases of the variant have gone up by 5,472 to 12,431, according to Public Health England (PHE).
PHE officials said on Thursday that the Delta variant has now overtaken the Alpha (Kent) one as the most dominant in the UK.
They also warned that there is "early evidence to suggest there may be increased risk of hospitalisation" from Delta, but further data is needed to confirm this.
The most affected areas remain Bolton, where cases have risen by 795 to 2,149, and Blackburn with Darwen, which has seen 368 new cases, bringing them to 724 in total.
There are encouraging signs that transmission rates in Bolton are falling and that actions such as increased mobile testing have been successful in reducing the spread, according to PHE.
Earlier on Thursday, Transport Secretary Grant Shapps said that the decision to move Portugal from the green to amber travel list was down to concerns about the Nepal mutation of the Indian variant.
As a result, Portugal's positivity rate has "nearly doubled since the last review", he said.
Dr Jenny Harries, chief executive of the UK Health Security Agency said of the new numbers: "With this variant now dominant across the UK, it remains vital that we all continue to exercise as much caution as possible.
"The way to tackle variants is to tackle the transmission of COVID-19 as a whole. Work from home where you can, and practice 'hands, face, space, fresh air' at all times.
"If you are eligible and have not already done so, please come forward to be vaccinated and make sure you get your second jab. It will save lives."
PHE also said outbreaks and clusters of the Delta variant in primary and secondary schools are at low levels, but there has been a slight increase over recent weeks, in line with higher levels circulating in the community.
The latest PHE data suggests there have been 97 confirmed coronavirus 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.
Analysis: No reason to hit the panic button - the vaccine is holding up well
By Thomas Moore, science correspondent
There is still no reason to hit the panic button over the Indian, or Delta, variant.
New data from Public Health England showing that people who are confirmed to have the virus are more than twice as likely to need hospital admission is certainly a concern.
But reassuringly the vaccine is holding up really well.
Of 89 people who were admitted to hospital with the variant, 59 - that's two thirds - had not been vaccinated at all. Just three of the patients had been double vaccinated.
The Delta variant, which is thought to be more infectious than previous strains, is adding to the rise in cases that had been expected as the UK came out of lockdown.
But the rise in hospital admissions has been small in comparison - and there has been no uptick at all in patients needing a ventilator.