The R value could be above 1 in England for the first time since late January, the Scientific Advisory Group for Emergencies (Sage) said on Friday.
Sage said the coronavirus reproduction number, or R value, in England is between 0.8 and 1.1, up slightly from last week's figure which was between 0.8 and 1.
R represents the average number of people each Covid-19 positive person goes on to infect and is seen as a key indicator measuring if the pandemic is under control or not.
An R number between between 0.8 and 1.1 means that, on average, every 10 people infected will infect between 8 and 11 other people.
The indication that R may be slightly above 1 could be a problem for the government and devolved nations, which are all planning to lift more lockdown measures next month.
What the R rate is in each region of England
In England, the R rate is 0.8 to 1.1
East of England – 0.8 to 1.1 (unchanged)
London – 0.8 to 1.1 (unchanged)
Midlands – 0.7 to 1.0 (up from 0.7 to 0.9)
North-east and Yorkshire – 8 to 1.0 (up from 0.7 to 1.0)
North-west – 0.7 to 0.9 (unchanged)
South-east – 8 to 1.0 (up from 0.7 to 0.9)
South west – 8 to 1.2 (up from 0.7 to 1.1)
The last time Sage said R was above 1 in England was on January 29 just as the third wave of Covid began to pass its peak.
The highest it ever reached was between 1.0 to 1.4 on January 8.
Despite the warning from Sage, the latest figures from the Office For National Statistics show the number of Covid cases is falling across the UK.
The ONS estimated on Friday around one in 1,010 people in private households in England had Covid-19 in the week to April 24 – down from one in 610 the previous week.This is the lowest figure since the week to September 5 last year when the estimate stood at one in 1,400.Although an increase in the R value is a worry for the government the stats from the ONS show almost no people over the age of 70 are catching the virus.
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With almost all over 70s fully vaccinated, an increase in cases may not be met with an increase in hospitalisations and deaths - giving the government more room to lift lockdown.
The age group most likely to catch Covid-19 is still secondary school children.
Sage said the growth rate, which estimates how quickly the number of infections is changing day by day, is between minus 4% and minus 1% for England.
It means the number of new infections is shrinking by between 1% and 4% every day.
Sage has said that R estimates now span 1 for England and some NHS England regions.
It added these estimates do not necessarily mean R is definitively above 1 and that the epidemic is increasing, just that the uncertainty means it cannot be ruled out.
The ONS said Yorkshire and the Humber had the highest proportion of people of any region in England likely to test positive for coronavirus in the week to April 24 and is the only place where their case numbers are not dropping.