The reproduction number, or R value, of coronavirus transmission across the UK is now above 1.
Figures published by the Government Office for Science and the Scientific Advisory Group for Emergencies (Sage) estimate the figure to be between 1.1 and 1.2.
Last week, the R number was between 0.9 and 1.
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.
An R number between 1.1 and 1.2 means that, on average, every 10 people infected will infect between 11 and 12 other people.
The slight increase comes as Boris Johnson has so far refused to follow the likes of Wales and Northern Ireland by ordering another lockdown for England.
The estimates for R and the growth rate are provided by the Scientific Pandemic Influenza Group on Modelling (SPI-M), a sub-group of Sage.
The growth rate, which estimates how quickly the number of infections is changing day by day, is between plus 1% and plus 4% for the UK as a whole.
It means the number of new infections is growing by between 1% and 4% every day.
Estimates for R and growth rates are shown as a range and the true values are likely to lie within this range, according to the experts.
Sage also said the figures published on Friday more accurately represent the average situation over the past few weeks rather than the present situation.
The R estimate for England has increased compared to last week and is between 1.1 and 1.3, Sage said.
Experts advising the Government have revealed all NHS England regions have R estimates that are above or span 1, suggesting the epidemic is growing in much of the country.
The East of England, South East and London now have R estimates well above 1, they said.