Labour leader Keir Starmer has called for Boris Johnson to urgently review the Covid rules for Christmas amid a sharp rise in cases. 

In a letter to the prime minister, Starmer called for Cobra to be convened “in the next 24 hours”, claiming the government has “once again lost control” of the coronavirus pandemic. 

It comes amid growing fears the five-day relaxation of the rules between December 23 and 27 could see the pandemic spiral in January, when winter pressures on the NHS are at their most acute. 

Meanwhile, London, most of Essex and parts of Hertfordshire are set to move into the strictest local lockdown, tier 3, from Wednesday. 

The letter reads: “It has become increasingly clear over recent days that the tier system you introduced two weeks ago has failed to control transmission of Covid-19. 

“Sadly, it does now appear that the government has – once again – lost control of infections, putting our economy and our NHS at grave risk in the new year.”

He adds: “I understand that people want to spend time with their families after this awful year, but the situation has clearly taken a turn for the worse since the decision about Christmas was taken. It serves no-one for politicians to ignore this fact. 

“It is my view that you should now convene Cobra in the next 24 hours to review whether the current relaxation is appropriate given the rising number of cases.”

Former Tory health secretary Jeremy Hunt has also called for Johnson to review the rules. 

He told BBC Radio 4′s World At One: “To go into the start of January, which I remember from my own time as health secretary is always the nightmare period for the NHS, with the same number of beds taken up by Covid as was the case in the peak this April would be a very, very dangerous and precarious situation.”  

He added: “We need to think about what the public would really want because the economic impact could be worse from taking our foot off the pedal for Christmas if it meant that we then had to go into another national lockdown.” 

Three households can be together for five days between December 23 and 27 regardless of what tier the area is in or whether people have self-isolated in advance. 

Asked directly on Tuesday if Downing Street was looking at cutting the Christmas period for exemptions from five days to three, or cutting bubbles from three households to two, the prime minister’s official spokesperson said it would keep regulations under review.

He said: “We have set out the guidance for Christmas bubbling arrangements but we obviously keep all advice under constant review.”

In a joint editorial the British Medical Journal (BMJ) and Health Service Journal (HSJ), have also warned Johnson is set to make a “major error” if he does not reverse plans. 

Starmer said that Labour would back the government if it moved to tougher restrictions over Christmas in order to avoid a second national lockdown. 

“If you conclude with government scientists that we need to take tougher action to keep people safe over Christmas, then you will have my support,” the letter said.  

Warning that a failure to act could cause long-term damage to the NHS and economy, Starmer also said the decision over Christmas was “a critical moment for our country”. 

“The tiered system has not kept the virus under control and has left us with precious little headroom,” he said. “Put simply, if you take the wrong decision now, the ramifications for our NHS and our economy in the new year could be severe.”