This means that, on average, every 10 people who test positive for coronavirus will go on to infect between 11 and 13 more people.
These estimates, which have increased from the previous range of 1.1-1.2, are based on the latest data available up to 18 December.
The growth rate, which reflects how quickly the number of infections is changing day by day, is estimated to be between one per cent and six per cent.
If the growth rate is greater than 0, then the epidemic is growing. If the growth rate is less than 0, then the epidemic is shrinking.
The Scientific Advisory Group for Emergencies (Sage), which publishes the figures, said the R and growth rate are “averages over very different epidemiological situations and should be regarded as a guide to the general trend rather than a description of the epidemic state."
It added: “There has been a divergence in policies across the four nations that constitute the UK, so the estimate of R for the entire UK has become less meaningful in recent weeks.”
Scientists use several different models, each drawing data from a variety of sources, to estimate the nationwide R and growth rate.
These include data such as the number of people testing positive, hospitalisations and deaths.
The period between initial infection, developing symptoms, the need for hospital care, and death means that these data can take up to three weeks to fully reflect changes in viral transmission.
The Cabinet's Covid operations committee met on Wednesday to consider the latest data on the spread of the virus.
Communities secretary Robert Jenrick acknowledged "it may be necessary to take further action" to curb rising case numbers.
The current tier levels in England are due to be reviewed on 30 December but that could be accelerated due to fears about the spread of the new variant, which appears to be transmitted more readily than the original form of coronavirus.
Mr Jenrick told Sky News: "We keep this under review, we are constantly hearing from our scientific advisers about what we should do."
The new variant is "very concerning" and is "prevalent probably in most regions of the country", he acknowledged.
Mr Jenrick said if the plans needed to change due to the new variant "we won't hesitate to do so".
But later he told BBC Radio 4's Today programme that there will not be changes to the Christmas arrangements.
"We are not going to change people's plans 24, 48 hours ahead of Christmas," he said, but "the strong advice is to keep it small, to keep it short and therefore to be safe".
Genomic researchers have found that the new variant, which is said to be 70 per cent more infectious, has already spread around the UK, with cases identified in Wales and Scotland.