England's R number has jumped to between 1.2 and 1.4, with the North West the worst affected, according to new figures.
R represents the average number of people each person with COVID-19 goes on to infect, so every 10 people with COVID this week will go on to infect between 12 and 14 other people.
Last week, the estimated R number was between 1 and 1.2 in England.
An outbreak can grow exponentially when the figure is above 1. When it is below 1, it means the epidemic is shrinking.
The rate at which cases are growing in England is between 3% and 6% per day, an increase on last week's daily growth rate, which was 0% to 3%.
The North West has the highest R number and growth rate, while the North East and Yorkshire has the lowest.
These are the regional R numbers (first) and growth rates (second):
East of England - R: 1.2 to 1.4 / Growth: 3 to 6
London - R: 1.1 to 1.4 / Growth: 2 to 6
Midlands - R: 1.1 to 1.3 / Growth: 1 to 5
North East and Yorkshire - R 1 to 1.2 / Growth: 0 to 4
North West - R: 1.3 to 1.5 / Growth: 4 to 8
South East - R: 1.1 to 1.4 / Growth 1 to 6
South West - R: 1 to 1.3 / Growth: 0 to 6
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NHS England said particular care should be taken with numbers from the South West as they are based on low numbers of cases, hospital admissions or deaths, and/or dominated by clustered outbreaks, and so may not reflect the entire region accurately.
The North West has been hit the worst by cases of the Delta (Indian) variant, which Public Health England (PHE) revealed on Friday is 64% more transmissible indoors than the Alpha (Kent) variant.
Vaccines are also less effective against the variant, with efficacy dropping by 17% with the first dose but only a minimal fall with the second, prompting scientists to urge everyone to get their second dose when they can.
In the North West, 45.1% of its 14,992 cases up to 7 June were identified as the Delta variant - far higher than the second-highest region, which is London at 15.3% of its 5,086 cases.
A total of 1.4% of the South West's 464 overall cases were the Delta variant, the lowest proportion.
It was revealed yesterday that 91% of new COVID cases in the UK are the Delta variant.