The reproduction number, or R value, of coronavirus transmission across the UK is between 1 and 1.4, the Government Office for Science and the Scientific Advisory Group for Emergencies (Sage) said.
When R was last updated on December 23 2020, it was between 1.1 and 1.3.
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 and 1.4 means that, on average, every 10 people infected will infect between 10 and 14 other people.
Sage has said the estimates published on Friday represent the transmission of Covid-19 over the past few weeks rather than the present situation.
This is due to the time delay between someone being infected, having symptoms, and needing healthcare.
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 0 and plus 6% for the UK as a whole.
It means the number of new infections is growing by between 0 and 6% 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.
Scientists advising the Government said the estimates of R and growth rates are based on the latest data available up to January 4.
Sage said: “R is a lagging indicator and so these estimates cannot account for the impact of recent policy changes or changes in transmission that have not yet been reflected in epidemiological data.
“This includes any changes that might have occurred over the festive period or the lockdown in England, announced on 5 January.”
The experts said that the novel coronavirus variant is growing much faster than other variants currently circulating in the UK, and it will take more time to fully understand the full impact of the new variant on the R number.
