Coronavirus infections are doubling every nine days as the second wave of the virus hits a "critical" stage, according to a major study.

Interim results from an Imperial College London study of 86,000 volunteers in England revealed infections were spiralling across the country, with the virus reproduction rate - known as R - at nearly 1.6 last week.

This was up from 1.16 in the previous round between September 18 and October 5.

An R number between 1.2 and 1.5 means that on average every 10 people infected will infect between 12 and 15 other people.

Researchers said early signs were emerging that areas with previously low rates of infection were now following trends seen in the worst-affected regions.

Experts have warned the second wave of the pandemic is at a 'critical' stage

They warned that the situation needs to change before Christmas, with more stringent measures needed sooner rather than later as the current restrictions are "not sufficient".

It comes as the UK death toll topped 300 for the second day running.

And new restrictions mean around 8.7 million people in England will be living under Tier 3 lockdowns from Friday when Nottinghamshire moves into the highest alert level.

Analysis of swab results from 86,000 people taken between October 16-25 show there are around 96,000 new infections per day.

Overall prevalence of infection was 128 people per 10,000, up from 60 per 10,000 in the previous round, the study found.

Infections are doubling every nine days, with interim estimates of R above 2 in the South East, East of England, London and South West, the data suggests.

The experts warned: "The co-occurrence of high prevalence and rapid growth means that the second wave of the epidemic in England has now reached a critical stage.

"Whether via regional or national measures, it is now time-critical to control the virus and turn R below one if further hospital admissions and deaths from Covid-19 are to be avoided."

While some of the effects from new lockdown measures may not have filtered through yet, the experts found the prevalence of infection was highest in Yorkshire and The Humber at 2.72%, up from 0.84%.

In the North West it was at 2.27%, up from 1.21%, and lowest in the East of England at 0.55%, up from 0.29%.

According to the study, prevalence increased across all age groups with the greatest rise in those aged 55-64 at 1.20%, up from 0.37%.

Clusters of Covid-19 cases were most prevalent in Lancashire, Manchester, Liverpool and West Yorkshire, West Midlands and East Midlands.

Professor Paul Elliott, Director of the REACT Programme at Imperial, said: “These interim findings paint a concerning picture of the situation in England, where we’re seeing a nation-wide increase in infection prevalence, which we know will lead to more hospitalisations and loss of life.

“We’re also detecting early signs that areas which previously had low rates of infection are following trends observed in the country’s worst-affected areas.

“Now more than ever we must all work together to curb further spread of the virus and avoid subsequent overwhelming of the health service.”

Coronavirus cases continue to rise across the UK

Steven Riley, Professor of Infectious Disease Dynamics at Imperial, said: "Our data show that there's absolutely no reason to expect that exponential rise to change to the next few weeks, regardless of what we do, because we're measuring infections occurring in the community."

He said it was not up to him to say what further measures may be introduced, but added: "If we are going to consider at some point over the winter that we may have to do something much more stringent, then it becomes a question of time.

"I think these results do argue for something sooner rather than later."

Prof Riley said: "There has to be a change. The rate of growth that we're seeing and in these data is really quite rapid, so one way or another there has to be a change before Christmas.

"We've fairly reliably measured a slight decrease in R in our interim round five, now we have measured a slight increase in R, and the slight increase in R means that current measures are not sufficient."

The data is released as a pre-print and has not been peer-reviewed.