It means that on average every 10 people infected will go on to infect between 8 and 11 other people, meaning cases could still be falling - but it may also indicate a slight increase too.
Sage, which provides the estimates, said that R now spans 1 for some England regions.
However, 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.
“This does not necessarily mean R is definitively above 1 and that the epidemic is increasing, but that the uncertainty means it cannot be ruled out,” it said.
This is the lowest figure since the week to September 5 when the estimate, which is based on a sample of the population, stood at one in 1,400.
The figures relate to people living in private households and do not include care homes, hospitals and prisons.
In Wales, around one in 4,230 people are estimated to have had Covid-19 in the week to May 8 – down from one in 2,070 in the previous week.
In Northern Ireland, the estimate is around one in 1,430 people, down from one in 750.
The estimate for Scotland is around one in 1,250, down from one in 760 and the lowest since estimates began for Scotland in October.
The figures come amid concerns grow over the Indian variant in the UK, with speculation it could push back June's final relaxation of lockdown restrictions.
Additional reporting from the Press Association.