A group of 70 backbench Tory MPs are threatening to block Boris Johnson's plans to tighten the tiered system of local lockdowns, which England will return to after its lockdown ends.
The MPs, members of the newly formed Covid Recovery Group, say they will not support the prime minister's proposals for stricter rules unless it’s proven they will "save more lives than they cost".
Mr Johnson is expected to outline on Monday his plan for England to return to the three-tier system when lockdown ends on December 2, but with a stricter set of rules designed to keep down coronavirus rates ahead of a relaxation at Christmas.
When the tiers return it is thought more areas will be placed under the tightest set of restrictions and the controversial 10pm curfew on pubs and restaurants will be altered.
Mr Johnson is expected to say that, while last orders must be called at 10pm, people will get an extra hour to finish their food and drinks with opening hours extended to 11pm.
But the group of backbench Tories say they will vote against the PM's plans if the government doesn't publish a "full cost" economic analysis of the restrictions.
In a letter to the prime minister, they said: "Government must publish a full cost-benefit analysis of the proposed restrictions on a regional basis so that MPs can assess responsibly the non-Covid health impact of restrictions, as well as the undoubted impact on livelihoods."
The letter added: "Restrictions should be removed immediately if it cannot be shown that they are saving more lives than they cost."
Chancellor Rishi Sunak on Sunday suggested providing the evidence they require will be a tall order, paving the way for a significant challenge to get Parliament’s approval for the restrictions.
The group of 70 MPs could inflict a defeat on Boris Johnson's government in the Commons, but they would need the support of most opposition MPs, which is not likely to be forthcoming.
Labour has backed all of the government's coronavirus restrictions and is likely to continue.
Nevertheless, the prime minister will be hoping he can persuade the rebels to back him, so the passage of his proposals does not rely on Labour votes.
The plan will set out how people will be able to spend their Christmas, but ministers have made clear that the festive season will be different to normal, with some restrictions expected to remain in place.
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Ministers will set out what tier each area will be placed into on Thursday and MPs are expected to be given the vote to approve the new tiering system, as promised by Mr Johnson, in the days before it comes into force on December 2.
The government is optimistic that restrictions can be gradually reduced in the run-up to spring, providing vaccines are approved by regulators, allowing a plan for the rollout to begin next month before a wider programme in the new year.
The Covid Recovery Group, led by former chief whip Mark Harper and former Brexit minister Steve Baker, said "the lockdown cure prescribed runs the very real risk of being worse than the disease".
"We are especially concerned about outside sport, the 10pm curfew, closure of non-essential retail, gyms and personal care businesses, restrictions on worship, care home visits, hospitality and the inclusion of children under 12 in the 'Rule of 6'.
"In these areas, we need to be assured of the evidence of their effectiveness, whether a mandatory approach produces better outcomes than a voluntary one, whether a blanket approach produces better outcomes than a targeted one, whether adherence will be sustainable and crucially, that transparent assessment shows they do more good than harm.
"Where harms are certain but benefits uncertain – such as closing schools – an intervention should not be used. The burden is on the government to demonstrate the necessity and proportionality of each restriction." The letter went on: "Our country instead needs a different and enduring strategy for living with the virus that can last beyond Christmas. Restrictions should be removed immediately if it cannot be shown that they are saving more lives than they cost."
A Number 10 spokesman said: “We understand that colleagues have concerns and we will continue to engage with them to address the issues.
“The Prime Minister has been clear that we are committed to minimising damage to our economy, lives and livelihoods.
“In the coming week, the Prime Minister will set out how advances in vaccinations, treatments and mass testing will help enable life to gradually return to normal but the virus is still present and without regional restrictions we risk putting in jeopardy the progress the country has made over the past few weeks.”