Boris Johnson is under growing pressure to further tighten coronavirus rules across England amid fears a new variant is accelerating the spread of the disease.

Former health secretary Jeremy Hunt warned the situation was on a “knife-edge” with infections rising and hospital admissions on the increase.

The Prime Minister held an unscheduled meeting on Friday evening to discuss the latest evidence about the mutant strain amid “growing concern” in No 10.

Earlier, he refused to rule out the prospect of a third national lockdown for England if infection rates continue to grow – a move he has previously been deeply reluctant to contemplate.

Wales and Northern Ireland have already announced fresh lockdowns once Christmas is over while the Scottish Government has said the option “remains on the table”.

Much of the the latest focus is on the South and East of England where the new variant is thought to be most prevalent.

On Friday, the NHS in Kent announced it was suspending non-urgent hospital procedures, saying it was treating double the number of patients it had at the peak of the first wave in April.

The move came as the latest figures from the Government’s Scientific Advisory Group for Emergencies (Sage) showed the R number for the UK was estimated to have risen to between 1.1 and 1.2 – which means the disease is growing again.

At the same time, there are concerns about a possible upward spike in cases as rules on households mixing are relaxed over the Christmas period.

Mr Hunt, who now chairs the Commons Health and Social Care Committee, said that if the Prime Minister did not want to change the regulations, he should at least consider issuing stricter guidance.

He said the prospects of a new year lockdown in England were on a “knife-edge”.

“It is a very difficult, finely-balanced judgment. The biggest worry is what happens indoor in family gatherings and that’s where the risks do increase,” he told the BBC Radio 4 programme.

“I think they can be clearer about what is and isn’t advisable because it would be an enormous tragedy if we had a spike in deaths at the end of January/February because we took our foot off the pedal this close to having a vaccine.”

Professor Sir Mark Walport – a Sage member and a former chief scientific adviser to the Government – said there was a real possibility the new variant could have a “transmission advantage” enabling it to spread more rapidly.

He warned the country was heading into the new year in a “serious situation”.

“Scientists are working extremely hard to work out what is going on. But it does definitely seem possible that this (new strain) transmits more easily,” he told the BBC.

“Even in the parts of the country where the infection is coming under control the cases remain stubbornly high so the NHS is under great pressure.

“We are going into January in quite a serious situation where we do need quite strong measures to socially distance and if the virus is changing then that makes it even more imperative.”

Downing Street would not comment on reports that among the measures being considered by ministers were new travel restrictions for the South East of England.

Friday’s meeting came as large parts of the region were following London and large parts of Essex and Hertfordshire, which entered the toughest Tier 3 restrictions earlier this week.

The latest tiering changes in England – which came into force on Saturday morning at one minute past midnight – mean 38 million people are now living in Tier 3, 68% of the population of England.

Those areas moving into the highest tier are Bedfordshire, Buckinghamshire, Berkshire, Hertfordshire, most of Surrey, Peterborough, Hastings and Rother on the Kent border of East Sussex, and Portsmouth, Gosport and Havant in Hampshire.

Bristol and North Somerset have moved down from Tier 3 to Tier 2 while Herefordshire drops into Tier 1 – even though the county’s public health director raised concerns about the relaxation.