Image caption An overhaul of the NHS was announced in July

The government has rejected a warning by doctors' leaders that its planned health reforms could undermine the long-term future of the NHS in England.

It set out plans in July to give GPs control of much of the budget, while scrapping two tiers of managers.

The British Medical Association said it was not against the whole vision, but had concerns the changes could affect the service's "stability and future".

Health Secretary Andrew Lansley said it was right to proceed with the reforms.

He said: "There are many GPs across the country who are keen to make the transition quickly, others want to know more about how it's going to work before they implement it," he said.

"This is what the consultation process is about, everyone coming forward to say how can we make this work."

He added that the plans were aimed at making care better for patients.

But the BMA's warning sparks the start of what promises to be a delicate balancing act for ministers.

The union's criticisms - made in its official response to July's White Paper - contrast with its initial response over the summer when it said it was "ready, willing and able".

And they come as the government faces a legal challenge from the union Unison.

The public sector union is seeking a judicial review over the way the government is handling the changes.

The official consultation period will end later this month, after which ministers are likely to start formal talks with BMA negotiators about implementing the changes.

The government wants to start piloting the GP consortiums, which will take charge of the budget from the soon-to-be abolished primary care trusts, this financial year. Full roll-out will follow within two years.

But it is this pace of change which is one of the problems, according to the BMA.

The key NHS changes GPs - Asked to get together in groups to take on responsibility for spending much of the NHS budget

- Asked to get together in groups to take on responsibility for spending much of the NHS budget Hospitals - Encouraged to move outside the NHS to become "vibrant" industry of social enterprises

- Encouraged to move outside the NHS to become "vibrant" industry of social enterprises Patients - More information and choice, including ability to register with any GP they want to

- More information and choice, including ability to register with any GP they want to Managers - Strategic health authorities and primary care trusts facing the axe

It said the timetable could threaten the £20bn savings the health service has to make by 2014.

The BMA also took issue with the "obsession" with competition, saying GPs would be set against hospitals - one of the objectives of the changes is to get more care done more cheaply outside hospitals.

Private sector

Many doctors also fear the plans will lead to the increased involvement of the private sector - and ultimately damage the doctor-patient relationship as the public could start viewing their own GPs as rationers of services.

The BMA said there needed to be a clear distinction between individual GPs and the consortiums.

Dr Richard Vautrey, deputy chairman of the BMA's GPs committee, told BBC Radio 4's Today programme that his organisation had "serious concerns".

Analysis: Does it matter what the BMA thinks? The BMA is probably one of the most powerful unions. But it has carefully positioned itself above the normal rough and tumble of the labour movement. In fact, it is just as likely to throw its weight around in debates about drinking and science as it is over pay and conditions. And with close links to influential voices across the political spectrum, it has a significant powerbase. But that does not mean it has a veto on these changes. While the government is likely to seek formal talks soon, ministers could force through the plans if they wished. Of course, that would create resentment within the profession and for the reforms to work the government needs doctors on side. Instead, expect a long and drawn-out period of negotiations, claim and counter-claim with today's intervention just the start.

He added: "Patients want to see their doctors and other healthcare professionals, in general practice and in hospitals, collaborating with one another - not competing with one another."

BMA chairman Dr Hamish Meldrum said that there "were proposals in the White Paper that doctors can support and want to work with".

He added: "But there is also much that would be potentially damaging.

"The BMA has consistently argued that clinicians should have more autonomy to shape services for their patients, but pitting them against each other in a market-based system creates waste, bureaucracy and inefficiency."

However, some doctors remain supportive of the planned changes, such as Dr Simon Brown, of Huntingdon, Cambridgeshire, who is taking part in one of the pilot schemes.

He told BBC Breakfast that control of the NHS's budget and purchasing decisions would benefit from the "clinical input" that doctors bring, something he said "had been largely absent until now".

Dr Brown added: "I'm confident that actually we can introduce some changes that will make a very positive impact on my patients."

Shadow health secretary Andy Burnham said: "This backs up Labour's warnings since the White Paper was published."

He warned that, if Health Secretary Andrew Lansley pressed on with his plans, he could expect the "fight of his life".

And Jennifer Dixon, chief executive of the Nuffield Trust, a health think tank, suggested the BMA was right to highlight some of the issues.

Media playback is unsupported on your device Media caption Andrew Lansley: 'This is a real opportunity'

"I think there is quite a lot of worry in the health service that these changes are too much too fast," she said.

"The principle of giving GPs, or doctors, budgets to manage is a quite good one, I think there is a lot of agreement about that.

"But the speed at which these changes are occurring - the fact that £70bn of public funds is going to be given to practices who are not used to handling this amount of money - is, I think, worrying."