The coronavirus R rate could have climbed to 1 across the UK - and may be higher than this in at least three regions.
London, the North West and the South West are all estimated to have R rates that could be above 1 - meaning the number of new cases is on the rise.
Nationally the reproduction rate is estimated to be between 0.8 and 1, government figures published this afternoon reveal.
The R rate - which refers to the number of people infected by each person with Covid-19 - needs to be below 1 in order for the number of cases to fall.
Despite cause for concern in some regions, the overall number of cases could be falling by up to 5% across the UK, data shows.
In the capital, the current R rate is estimated to be between 0.8 and 1.1.
This is the same estimated range as the North West, a week after Greater Manchester and parts of Yorkshire and Lancashire had new lockdown restrictions imposed.
The government has previously warned that if it sees regional spikes, it could ban travel between cities in order to prevent a second national lockdown.
Earlier this week worrying data revealed 17 of London's 32 boroughs saw increases in the infection rate in the week to July 31 - with the rate trebling in Barking and Dagenham.
An R number of 1 means that on average every person who is infected will infect 1 other person, meaning the total number of infections is stable.
If R is 2, on average, each infected person infects 2 more people.
If R is 0.5 then on average for each 2 infected people, there will be only 1 new infection. If R is greater than 1 the epidemic is growing, if R is less than 1 the epidemic is shrinking.
The government says: "A growth rate between 0% to -5% means the number of new infections is somewhere between remaining stable and shrinking by 5% every day.
"The UK estimates of 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."
