The reproduction number, or R rate, of coronavirus transmission across the UK has risen to between 1.1 and 1.2, according to the latest government figures.
It represents a significant increase from last week’s estimate of between 0.9 and 1.
R measures the number of people, on average, that each sick person will infect.
If R is greater than 1, the epidemic is generally seen to be growing; if R is less than 1 the epidemic is shrinking.
The estimate represents the situation over the past few weeks rather than a snapshot of the situation on Friday, due to the time delay between initial infection, symptoms appearing, and the need for hospital care.
Boris Johnson on Friday refused to rule out a third national lockdown in England in the New Year and warned the “reality” was the number of infections was rising.
The Government Science Office has also published regional R rates across England as supplied by NHS England.
Regional R numbers across England
England: 1.1 - 1.3
East of England: 1.2 -1.4
London: 1.1 -1.3
Midlands: 1.0 - 1.2
North East and Yorkshire: 0.9- 1.1
North West: 0.9- 1.1
South East: 1.1 -1.3
South West: 0.9 -1.2
The Scottish government currently estimates the R rate in Scotland as being between 0.9 and 1.1.
In Wales the latest estimate is that the R rate is between 0.8 and 1.1.
And the R rate in Northern Ireland is currently estimated at between around 1.1 and 1.2.
Northern Ireland will enter a tough six week lockdown beginning on Boxing Day in an attempt to suppress a surge in Covid-19 cases.
Asked on Friday if England could soon follow, the prime minister said: “We’re hoping very much that we will be able to avoid anything like that.
“But the reality is that the rates of infection have increased very much in the last few weeks.”
It comes as the Office for National Statistics (ONS) said the proportion of people testing positive is estimated to have increased sharply in London.
Infection rates have also increased in Eastern England, the East Midlands and south-east England, the ONS said.
The East Midlands has the highest rate (with an estimated 1.4% of people in private households testing positive for Covid-19), followed by London (1.4%) and north-east England (1.2%). South-west England has the lowest rate (0.4%).