The NHS faces a "quiet crisis" after it emerged 43 family doctor practices have closed in the last year after vital top-up funding was cut.
Labour MP Emily Thornberry, who serves on the Health Select Committee of MPs, said the Government isn't doing enough to "stave off disaster" despite its ambition for a seven-day NHS.
Deprived inner-city areas, university towns and remote rural communities are thought to have been the worst hit by the Government phasing out the so-called minimum practice income grant (MPIG), cash given to surgeries with shortfalls thanks to botched funding allocations devised a decade ago.
Ministers have defended the decision, saying a new formula will reflect levels of poverty and sparsity, and that the majority of GP practices stand to gain from the reallocation of the money.
NHS bosses have admitted 98 surgeries were at risk as the funding was phased out over seven years, though commentators fear hundreds more will be affected.
In June, Health Secretary Jeremy Hunt announced a "new deal" for GPs in England, asking them to adopt seven-day working in return for a £750 million investment.
He pledged to recruit 10,000 new clinicians for primary care – including 5,000 more GPs - and set up a £10 million rescue fund for struggling practices.
But critics warn it is too late for some, and a separate "bail-out" pot of cash is not getting to all those affected.
In response to a written parliamentary question tabled by Ms Thornberry, the Department of Health revealed 43 GP surgeries in England who were receiving MPIG funding have closed their doors since April last year. The department pointed out the figure includes surgeries that have merged or been taken over.
The MP claims this is "just the tip of a much larger iceberg" since almost 3,000 practices nationally having relied on the funding. GPs are effectively small businesses and receive payment for reach patient they treat.
Ms Thornberry, MP for Islington South and Finsbury, said: “GP surgeries are the lifeblood of the NHS, delivering 90% of the care the NHS provides.
"But there is a quiet crisis brewing in primary care, especially in inner London, where there are dozens of practices teetering on the edge of collapse as a result of the Tories’ unfair funding changes.
“This funding went to about a third of all GP practices in the country, representing only the areas with the highest levels of need. But in my constituency almost three quarters of practices – 72% - relied on that financial support."
She went on to warn Health Secretary Jeremy Hunt's review of the funding formula to reflect the needs of deprived inner-city areas "isn’t enough to stave off disaster".
She said: "It’s likely that the new formula won’t take effect until 2017, and practices cannot be expected to tread water in the meantime.
“That’s why I’ve called on the Government to provide an urgent financial reprieve to these struggling practices.”
Sarah Wollaston, chairman of the Health Select Committee of MPs, said the NHS workforce was an area it could potentially examine, and suggested she would propose it to members when it reconvenes.
She said: "The principle of a seven-day NHS to reduce people having to go to hospital is a good one. But we don't want to distort the priorities of GPs. There needs to be flexibilities to recognise the differences in different parts of the country."
An NHS England spokesman said: “NHS England has a responsibility to ensure that every patient has access to a general practice.
"A practice may close for a number of reasons, including mergers to create group practices that offer a wider scope of services for patients.
"Whatever the reason, any practice considering closure must go through a formal process with NHS England to ensure patients continue to receive high quality general practice care.”