There is a lack of detail about where “efficiency savings” demanded by the government of the health service are being made, analysts have said.

Health secretary Andrew Lansley has said “every penny” of the proposed £20bn savings by 2015 - 4 per cent to 5 per cent of the annual NHS budget - will be reinvested in patient care and has insisted the efficiency drive “does not mean cutting services”.

But Professor John Appleby, chief economist at the King’s Fund think tank, told the BBC there was a lack of detail about the issue.

The Royal College of Nursing has also said it has seen no evidence from the government of where savings are being directly reinvested in patient care.

Financial results released this week suggest £5.8bn will be saved this year in the health service.

Mr Appleby said it seemed as if ministers were relying on the pay freeze and a cut in money given to hospitals to make a large chunk of the savings.

He said this could have the effect of not incentivising staff to work as hard as they had done, while for hospitals the cuts could see them deliver services less well.

“These would not be productivity savings, they would be cuts,” he added.

He said the NHS faced a “pretty impossible” job in continuing to make gains in the long-term.

The comments come as a survey for the Health Service Journal of chief executives of NHS hospital trusts found a quarter were planning cuts to services.

In total, 31 per cent of chief executives said their organisations were planning to expand in the next 12 months but 25 per cent said their hospitals would be cutting back on services offered to patients.

Many chief executives said they believe they will struggle to improve quality of care.

Asked to rate their confidence that their organisation could “improve quality of services over the next 12 months” on a scale of one to 10, 33 per cent gave a five or lower while 16 per cent gave a score of nine or 10.

David Stout, of the NHS Confederation, which represents managers, told the BBC that while the financial figures released this week were good news, they were probably down to the easier savings.

He said the “most challenging” actions, such as major changes to the way hospitals were run, still remained.

David Flory, deputy chief executive of the NHS, said the figures were “encouraging and show the NHS continues to deliver the best care for patients while maintaining a healthy financial position”.

He added: “It is vital that the NHS does not take its eye off the ball - the NHS is performing well to meet future challenges, and must continue to do so.”

Earlier this week, Mr Flory told the Financial Times the NHS will need to eke out substantial year-on-year savings for “as far ahead as we can see”.

It follows a revelation by the newspaper that the Department of Health has warned health service managers they have to find up to £50bn of “efficiencies” by the end of the decade.

Last week, Chancellor George Osborne was also accused by Labour of raiding the NHS budget as it emerged that planned spending by the health service has been reduced by £500m.

The move, disclosed in Treasury figures following last Wednesday’s Budget, follows a £900m underspend in 2011-12, only £400m of which has been rolled over into the 2012-13 budget.

The Department of Health said the underspend was down to greater than expected efficiencies in capital projects, including an IT scheme, but that the coalition’s commitment to increase the NHS budget in real terms was still being met.

But shadow health secretary Andy Burnham said the NHS was “taking a hit” to help fund the cut in the top rate of income tax for people earning more than £150,000 a year.