The reproduction number, or R value, of coronavirus transmission across the UK has risen above 1.
Data released on Friday by the Government Office for Science and the Scientific Advisory Group for Emergencies (Sage) shows the estimate for R across the UK is between 1.0 and 1.2.
It comes as the number of newly diagnosed cases continues to rise, with higher incidence seen in young people aged 18 to 24 years.
According to Government advisers, the last time R was above 1 was in early March.
They have described the figures as a “wake-up call”, saying additional measures such as local interventions may be needed to control the spread of the virus.
R represents the average number of people each Covid-19 positive person goes on to infect. When the figure is above 1, an outbreak can grow exponentially.
In England, R is between 1 and 1.2.
The figure is slightly higher in London and the North West – between 1.1 and 1.3 in both regions.
The estimates for R and growth rate are provided by the Scientific Pandemic Influenza Group on Modelling (SPI-M), a subgroup of Sage.
They say the numbers suggests the virus is now spreading widely across the country.
Meanwhile, data from Imperial College London, which is based on a cohort of 150,000 volunteers within a specific time frame, estimates R to be 1.7.
But this is different from the weekly official government R value produced by Sage which uses multiple data sources such as hospital admissions, intensive care unit admissions and deaths, and models to produce a consensus view of the likely R number over a longer time frame.
The growth rate of coronavirus transmission, which reflects how quickly the number of infections is changing day by day, has also increased slightly.
For the whole of the UK, the latest growth rate is between minus 1% and plus 3% per day.
The growth rate means the number of new infections is somewhere between shrinking by 1% and growing by 3% every day.
The most likely value is towards the middle of the range, experts advising the Government say.
But they also said R and growth rates are not the only important measures of the epidemic and should be considered alongside other metrics.
These include the number of new cases of the disease identified during a specified time period (incidence), and the proportion of the population with the disease at a given point in time (prevalence).
In the East of England, the R number is 0.9-1.2, while the growth rate is between minus 1% and plus 4%.
The R number in the Midlands is 0.9-1.1, while the growth rate in the region is between minus 1% and plus 3%.
In the North East and Yorkshire, the R value is 1.0-1.2, while the growth rate is between plus 1% and plus 5%.
In the South East, the R value in the region is 1.0-1.2, while the growth rate is between zero and plus 4%.
The South West has an R value of 0.9-1.2 and a growth rate of between minus 1% and plus 4%.
In London, the growth rate is between plus 2% and plus 4% while in the North West, the figure is between plus 2% and plus 5%.
Over the past few months, R and growth rate estimates have been less useful in determining the state of the epidemic as the number of coronavirus cases was low.
But as cases continue to rise, experts from Sage say they now have a better estimate of these figures and that “R should be taken seriously”.
Meanwhile, data published on Friday by Imperial as part of the React-1 study suggests cases in England were doubling every seven to eight days at the beginning of September.
And the latest figures from the Office of National Statistics show around 3,200 people per day were newly infected with Covid-19 between August 30 and September 5, up from an average of 2,000 people per day between August 19 and 25.