Sports promotion experts have called for government monitoring of exercise-on-prescription schemes.
Public health minister Tessa Jowell will return from her summer break to find proposals for a quality assurance system for such schemes on her desk.
Up to 400 projects where GPs 'prescribe' exercise for patients suffering from mild to moderate illness, such as diabetes mellitus, have been set up over the last decade.
Ms Jowell praised the projects in the public health white paper, Saving Lives: our healthier nation, and promised that the government's sports strategy, due this autumn, would promote exercise on prescription as a 'cost-effective alternative to prescribing long-term medication'.
But legal insurers have warned that doctors could be sued by patients who suffer injury as a result.
Dr Peter Schutte, Medical Defence Union deputy head of advisory services, said: 'The vast majority of GPs are not sufficiently expert in sports medicine to know what the full implications of exercise on an individual will be.'
Concerns about the quality of fitness instructors have led the MDU to warn doctors that referring patients to someone not registered with a statutory body conflicts with General Medical Council guidance.
Dr Tom Coffey, chair of Balham, Tooting and Wandsworth primary care group in south London, said a new framework was 'necessary' because health improvement programmes were increasingly focusing on exercise to combat conditions such as obesity and coronary heart disease.
Exercise England, formerly the Sports Council, has drawn up the national quality assurance framework for exercise prescription.
Chief executive Dr Andrew Craig said: 'The framework will give people liberal but robust guidance on how to conduct exercise referral safely, how to audit what we are doing now and how to set up new schemes.'
The document should 'clear up the worries' of the medical defence bodies, he added.
The framework proposes teams of inspectors based at NHS Executive regional offices to liaise with the organisers of local exercise referral schemes.
Dr Craig said the scheme should be 'self-funding', covering its costs by fees paid by patients and scheme organisers. He said he wanted the Department of Health to go further and 'benchmark training programmes for people to run these schemes'.
A Department of Health spokesperson said Ms Jowell would 'consider very carefully' the proposals.
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