The head of the Department of Health's strategy unit warns today that the impatience of ministers to see results from the billions of pounds of new investment in the NHS is threatening its survival.

Professor Chris Ham, hired three years ago by Alan Milburn, the Secretary of State for Health, to help to spearhead NHS reforms, says in The Lancet that changing big organisations such as the NHS is slow and unglamorous work that depends on motivating professionals and can only succeed step by step.

But the pressures of the electoral cycle and the demands from the public to see rapid improvement mean that ministers are drawn to radical solutions that evidence shows nearly always fail.

The intervention of so senior a figure in the public debate on the NHS will cause consternation at Westminster. It comes a day after the Audit Commission warned that the proliferation of government targets is threatening progress, echoing concerns from the Commission of Health Improvement.

Professor Ham says the failure of "grand... ambitions harboured by reformers" stems from their limited impact on clinical practice. Making a difference to the experience of patients depends on changing the day-to-day decisions of doctors and nurses.

But doctors and nurses cling to their professional autonomy, resisting demands for increased accountability, which makes them hard to manage. This increases the frustration of ministers and makes them more prone to resort to big bang changes.

Professor Ham writes: "If changes in health-system performance and clinical practice are best achieved through clinical engagement and by a series of incremental steps rather than through a big bang, then this process might not satisfy policymakers and taxpayers, who increasingly expect to see quick results. The risk then is that reformers will resort to the radical solutions that, at best, have had partial success. The result will be a widening gulf of understanding between politicians and clinicians."

He says "the impatience of reformers and those they serve" is one of the two major obstacles to progress, but he admits it is probably "heroic" to assume it can be overcome.

The other obstacle is the reluctance of professionals to accept the need for reform and to engage with it. "The days in which the role of governments was simply to provide the resources to enable professionals to practise autonomously have long gone, if they ever existed."

Financial pressures, rising public expectations and scandals of poor performance have increased public demands for accountability.

Professor Ham, one of the most experienced observers of the NHS over two decades of reform, was seconded to the Health Department in 2000 from his post as head of the Health Management Services Centre at the University of Birmingham, and has one more year of his contract to run.

He warns that significant change cannot be achieved without the co-operation of professionals and notes that the rejection of the proposed consultant contract by the NHS's most senior doctors is a reminder of how tenaciously they cling to their autonomy.

"The trick is to harness the energies of clinicians and reformers in the quest for improvements in performance that benefit patients... On this link, nothing less than the future of organised health-care systems depends."