Senior Conservatives and Liberal Democrats have warned the health secretary that huge swings in funding will force small, remote GP practices to close.
The loss of practices could prove fatal to rural communities facing cuts to schools, post offices and public transport, campaigners warn.
Foreign secretary William Hague, former Conservative party vice chairman Nigel Evans and Liberal Democrat party president Tim Farron have joined GPs and local campaigners demanding funds to maintain rural practices hit by MPIG reforms.
Mr Evans, MP for Ribble Valley, Lancashire, told GP the impact of MPIG changes 'had not been thought through'.
Under GMS contract changes imposed by the government in England, MPIG top-ups to core pay will be redistributed over seven years from 2014. About 65% of practices in England benefit from MPIG funding, with some receiving as much as half their core pay from the top-up.
GP reported earlier this year that the 100-patient Slaidburn practice (see picture below), in Mr Evans' constituency, could be forced to close just one year into the overhaul. The practice stands to lose £91,000 - a third of its total core pay and close to its entire annual profit - when MPIG is cut.
A significant number of practices across England could be in a similar position. The DH has estimated 100 practices in England could become unviable as MPIG is phased out.
Many may not be rural, but a 2006 report by consultancy Deloitte for NHS Employers said more than 300 practices in England were 10km or more from the nearest surgery.
Eighty of these had fewer than 1,900 patients and were likely to be 'unavoidably small', with above average costs because of their remote location.
Practices like Slaidburn with fewer than 1,200 patients received 'inducement' payments under the old Red Book GMS contract, which were in effect maintained by MPIG.
About a dozen GP practices have contacted Slaidburn single-hander Dr Karen Massey (picture below, centre) about joining her campaign for funding to maintain rural practices. Patients say the practice is the 'beating heart of the local community'.
Mr Evans said: 'This practice is doing amazing work in a relatively remote area - if (MPIG) is not replaced with some other funding mechanism that recognises the importance of a practice like Slaidburn, it will close.
'This is not a case of diminishing the service, it is destroying it, and will have a devastating impact on the local community.'
John Blackie, an independent councillor on North Yorkshire County Council and a member of the Yorkshire Dales National Parks Authority, has been lobbying MPs and councils to support rural practices.
Mr Blackie told GP: 'Without doctors in our midst, the future of areas like the Dales will be uncertain. If there is no local primary care, people will move away. That doesn't spell anything but doom and gloom for the community.'
He said GP practices, with schools, post offices and public transport, were the 'pillars that support local communities'.
It was vital to build 'a broadside of support across GP practices, patients, parish councils and other organisations to make NHS England think carefully and quickly about the savings', he argued.
Mr Blackie said he understood about £6m could be enough to protect the hardest-hit rural practices. This could be funded from the £170m total MPIG funding, he suggested.
The loss of one Yorkshire Dales practice, in Hawes, would leave patients with a 17-mile trip to the nearest GP, he warned. Elderly patients have become extremely worried by continuing doubt over their practices' future, he said.
GP visits could not be replaced with telehealth, phone or Skype consultations in communities where mobile reception and broadband were non-existent and some residents relied on shared 'party lines' for telephone calls, he added.
Mr Farron, MP for South Lakes, Cumbria, said he was 'seriously urging the government to reconsider these damaging policies and ease the pressure vital GP surgeries face'.
Mr Hague has written to Mr Hunt and NHS England, and assured local councillors: 'I take this issue seriously and am determined to help find a solution.'
The DH has yet to give any guarantees about the future of practices affected by MPIG cuts. An NHS England spokeswoman said it was 'committed to ensuring patients in rural areas can continue to access appropriate GP services', despite reforms to create a 'fairer system of funding'.
She said NHS England would work with the GPC on how to handle practices 'where MPIG makes up a significant proportion of income'. 'Different contracting arrangements may need to be considered,' she said.
GPC deputy chairman Dr Richard Vautrey said: 'It's ironic that you now have people like William Hague supporting these practices over problems created by their own government.'