Tory MPs in the North have demanded that Boris Johnson sets out a "clear roadmap out of lockdown" as they warn him he cannot leave people "with no end in sight".
The newly-established Northern Research Group (NRG) of more than 50 Conservative MPs wrote to the Prime Minister to make it clear that voters in "Red Wall" seats which the Tories they won from Labour last year will turn against him if they feel "forgotten".
More than eight million people are now living under Tier 3, the harshest level of England's restrictions, almost all of them in the North, with "greater clarity" needed on how they escape from the strict coronavirus measures.
The intervention is the most significant to date by the group, which is rapidly becoming a party within a party that has the power to overturn Mr Johnson's parliamentary majority.
The MPs, who include the former Cabinet ministers David Davis, David Mundell and Esther McVey, said they were becoming "increasingly concerned" that the Government will abandon its levelling up agenda to pay for the cost of Covid-19, and that their constituencies would be "left behind".
If the virus increases the "disparity" of the North-South divide, they say, "this would threaten to undermine the Government's hard-won mandate in December".
The MPs are alarmed that the Chancellor's decision to shelve his planned three-year spending review next month in favour of a one-year review could mean he will downgrade pledges to spend big on infrastructure plans for the North.
They called on Mr Johnson to "reaffirm our commitment to people living in the North with a Northern Economic Recovery Plan” that would provide jobs, investment and long-term prosperity.
First, however, the MPs want Mr Johnson to explain how regions will be taken out of the harsher restrictions.
The letter states: "We believe the Government can foster wider support by setting out a clear roadmap down the tiering system and out of lockdown. People want to see that measures will come to an end, and to feel reassured that the opportunities harnessed at the beginning of the year will be provided to them once again by this Government."
The emergence of the NRG is the biggest threat to Mr Johnson's authority since he came to power. Led by Jake Berry, a former Northern Powerhouse minister, it has the power to frustrate the will of the Prime Minister in the same way that the European Research Group was a constant thorn in the side of Theresa May.
If its members vote as a block against the Government in a full-scale rebellion and side with the opposition parties, they have the numbers to defeat Mr Johnson in the Commons.
Their biggest fear is that they will lose their seats at the next election in 2024 if people who "lent" their votes to Mr Johnson to get Britain out of the EU return to Labour.
Their letter adds: "Our constituents have been some of the worst affected by Covid, with many losing jobs and businesses. We urge you to reflect carefully on our promise to people living in the North during the last election with the levelling-up agenda and make our region central to the country's economic recovery.”
Mr Berry said on Monday night: "Our party's return to Government in December was won on the back of hard-working people in constituencies like ours who backed the Conservatives for the first time in a generation, and who did so on the promise that they would not be forgotten.
"We cannot forget that we must deliver on our commitments made during that election, to level up Northern communities and create opportunity across our region.
"The North has seen a level of disruption unparalleled with other parts of the country. The virus has exposed in sharp relief the deep structural and systemic disadvantage faced by our communities and it threatens to continue to increase the disparity between the North and South still further.
"Never has there been a more pertinent and urgent political and economic case to support people living in the North."
A Downing Street spokesman said: "We are absolutely committed to levelling up across the country and building back better after coronavirus.
"We stood at the last election on a solemn promise that we would improve people's lives, and although the pandemic has meant 2020 is not the year we all hoped it would be our ambitions for the country are unchanged."