Ending lockdown and reintroducing the tiered system could see infection rates soar once again, Sage experts have warned.

Scientists on the Scientific Advisory Group for Emergencies have said that returning to the previous tiered system could well cause infections to rise back to levels seen early in November.

Lockdown measures currently in place in England are likely to take the coronavirus R number below one, they say - but they are due to end on December 2.

And the possibility of returning to the previous tiered system in place before the second lockdown is not likely to be sufficient to keep the rate down, the scientists say.

A document from the group, dated November 4, said: “If England returns to the same application of the tiering system in place before November 5, then transmission will return to the same rate of increase as today.”

In a set of new documents released by the group on Friday, the experts said that if the four-week lockdown measures are well adhered to, the number of hospital admissions and deaths could fall until at least the second week of December.

But they said the longer-term outlook is less certain.

They say fortunes will depend on both the kind of non-pharmaceutical interventions implemented after December 2 along with Government policies over the festive period.

Another Sage document suggests the national outbreak is still at a “high and controlled” phase.

And this could have a major impact on all of us in the weeks ahead.

Sage says that if this remains the case, or if the outbreak goes back to current levels after the lockdown, there is “little to no scope for loosening of social distancing rules over Christmas”.

But if prevalence is “low and controlled” and R is “well below one” for some time, there may be a “greater potential for loosening of social distancing rules for a limited period of time during the festive period”.

The Government scientists also said that if R -  the reproduction number for coronavirus - is reduced to 1.1 or lower for some time, there may be a “limited accumulation of population immunity”.

Significantly, they say this will start reducing the average population susceptibility to the virus and slow down transmission.

Sage said: “When R is 1.1, only 9 per cent of the remaining susceptible (ie, not previously infected) population need to be infected for R to fall to one, solely as a result of the natural dynamics of the epidemic.

“At this point, in some sense, population immunity has caused the epidemic to plateau.”

However, they warned that population immunity is “very different from a classic ‘herd immunity’ scenario, where an epidemic has run through a population with limited impact of control measures”.

The experts said that in a population immunity scenario, there “will be very limited room to relax interventions, since the absolute level of population immunity reached will likely still be low”.

On October 12, the Prime Minister announced England would be placed into “medium”, “high” and “very high” alert levels – or Tier 1, 2 and 3 – involving varying restrictions aimed at tackling the virus.

But as the number of Covid infections continued to rise, a four-week lockdown was introduced on November 5.

The number of new daily cases across the UK is currently thought to be between 55,000 and 81,000.

Experts believe R is below one in some places, particularly in the north west of England.