85% of GPs have told a survey that the financial challenge facing the NHS will eventually force the government to set out more clearly what care is, and is not, freely available on the NHS in England.

The poll, conducted by Doctors.net.uk - the largest online network of doctors - and health policy think tank the Nuffield Trust, also found that 83% of GPs believe that handing responsibility to local clinical commissioning groups (CCGs) for setting priorities for spending NHS funds will be likely to lead to greater variations in what services are provided to patients throughout England.

In contrast, only half of GPs questioned believe that the NHS will be able to improve efficiency enough over the next five years to avoid having to scale back on the services that are currently funded.

The survey's findings are published alongside a report from the Nuffield Trust which examines the feasibility, as well as the advantages and disadvantages, of setting out explicitly the care patients are entitled to.

Recent political debates on the Health Secretary's duty to promote "a comprehensive health service" suggest there is considerable uncertainty in relation to how the NHS interprets national guidance at a local level, and therefore how much services reflect local need, says the Trust.

The study finds that drawbacks with the current system for determining which treatments are and are not funded by the NHS include a lack of transparency around how spending decisions are made and variations in funding decisions - the "postcode lottery" - resulting in perceptions of unfairness.

On the other hand, important current benefits are that the system allows doctors to respond to the needs of individual patients and enables commissioners to set local priorities within a fixed budget, it says.

The report does not recommend that the government should draw up an explicit account of what health care is or is not funded by the NHS - it rules this out for three main reasons.

First, say the authors, it would not be practically feasible to do so, given the cost and information requirements necessary to do it properly. Second, given the lack of information on which to base it, rigid application across a whole range of services for individuals would be inappropriate. Third, it risks unnecessarily compromising the principle of solidarity on which the NHS relies.

Instead, the report makes several recommendations for how the system could be improved so that, among other things, perceptions of unfairness could be avoided. These include:

- establishing a set of principles that would shape how public money is spent in the NHS and, conversely, inform decisions about what care and treatments will no longer be paid for. These principles could be enshrined in the NHS Constitution, and also restated in the annual Health Secretary's mandate to the NHS Commissioning Board - and, in turn to the new CCGs. This would remind NHS commissioners of what should underpin their decision-making about which serves to fund, says the Trust;

- producing a national list of the treatments that public money should not be spent on in the NHS - unless there are exceptional circumstances. Local commissioning groups should be expected to report on instances where they have diverged from national guidance; and

- ensuring decisions in CCGs are transparent and, where possible, clinicians are "nudged" towards clinical and cost-effective care, so that any proposed divergences from national guidelines and NHS commissioning principles would be subject to proper scrutiny before they are finalised.

The current system for defining what is in and out of the NHS "offer" is far from ideal, and, as the survey result show, a failure to get this right could lead to greater calls for defining this at national level, said Dr Judith Smith, head of policy at the Nuffield Trust.  This "would have significant drawbacks in terms of limiting the autonomy of local clinicians to make decisions based on what is best for their communities," she warned.

"Health care inequity is already a divisive issue and the changes in funding, with clinicians making decisions locally, will only serve to create more of a patchwork quilt," added Dr Tim Ringrose, chief executive of Doctors.net.uk.

"Minimum standards set out in guidance at a national level, and engaging doctors and trusting them to act in the interests of their patients locally, seems the right balance to strike," he said.