The UK's coronavirus R number, the rate at which it spreads, could be as high as 1.6 as experts warned the epidemic is still "growing exponentially" across the country.
New figures released on Friday by the Government Office for Science and the Scientific Advisory Group for Emergencies (Sage) show the estimate for R for the whole of the UK has risen to between 1.3 and 1.6 compared with last week when it was between 1.2 and 1.5.
Sage has cautioned that while there are some early indications that suggest the growth of the epidemic might be slowing, it is too early to draw firm conclusions amid emerging signs of a second wave of infections.
The Government's scientific advisers said “it is still highly likely that the epidemic is growing exponentially across the country” and more data is needed to accurately assess recent changes in coronavirus transmission.
“Over the next few weeks, it will be important that we understand this in the UK and do not become complacent,” they added.
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.
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 5 per cent and plus 9 per cent for the UK as a whole.
The most likely value is towards the middle of that range, according to the experts.
Scientists behind the data warned that as coronavirus case numbers rise, it will also be accompanied with a parallel increase in the number of deaths.
This is likely to continue for the next few weeks as mortality reflects the recent sharp increase seen in cases, they added.
The Sage scientists 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 Covid-19 cases identified during a specified time period (incidence), and the proportion of the population with the disease at a given point in time (prevalence), they added.
They also said the figures published on Friday more accurately represent the average situation over the past few weeks rather than the present situation.
Friday's developments came after Boris Johnson warned earlier this week that the UK was at a “critical moment” in the battle with coronavirus.
The Prime Minister used a Downing Street press conference on Wednesday to urge people to follow rules aimed at curbing the spread of the virus and demonstrate “forbearance, common sense and a willingness to make sacrifices for the safety of others”.
He warned that if the evidence required it “we will not hesitate to take further measures” that would “be more costly than the ones we have put into effect now”.
“But if we put in the work together now, then we give ourselves the best possible chance of avoiding that outcome and avoiding further measures,” he said.
Chief medical officer Professor Chris Whitty meanwhile said the number of people in hospital was rising, particularly in hotspots, although the figures remained at a much lower level than the beginning of April.
“We are pointing out that the direction of travel for both hospitals and intensive care is going in the wrong direction, particularly in these areas that have seen rapid increases in cases,” he said.
In a stark warning, Prof Whitty added: “We have got a long winter ahead of us”.