A watchdog has called on ministers to correct claims the coalition has increased NHS spending in England.
The UK Statistics Authority upheld a complaint by Labour about government claims the NHS budget had increased in real-terms in the past two years.
The watchdog found the best-available Treasury data suggested real-terms health spending was lower in 2011-12 than in 2009-10.
The coalition said during its spending review the NHS budget had gone up.
It said NHS spending had decreased in 2010-11 because of budget and spending plans set in place by the previous Labour government.
The watchdog has asked ministers to "clarify the statements made".
In 2010, the coalition said it would "guarantee that health spending increases in real terms in each year of the Parliament".
Labour asked the watchdog to look into claims made subsequently by the prime minister, the health secretary and others about meeting this commitment.
Andrew Dilnot, chair of the UK Statistics Authority, concluded that changes in NHS spending over the two years had been small and health spending was actually lower in 2011-12 than in 2009-10.
Mr Dilnot accepted that there were "questions of definition" and different sources of data.
But he said the watchdog's calculations were based on what he considered "the most authoritative source" of national statistics on the subject from the Treasury.
In a letter to Health Secretary Jeremy Hunt, Mr Dilnot said: "On the basis of these figures, we would conclude that expenditure on the NHS in real-terms was lower in 2011-12 than it was in 2009-10.
"Given the small size of the changes and the uncertainties associated with them, it might also be fair to say that real-terms expenditure had changed little over this period.
"In light of this, I should be grateful if the Department of Health could clarify the statements made."
Shadow health secretary Andy Burnham said it was evidence the government was cutting the NHS.
"For months, David Cameron's government have made misleading boasts about NHS spending, misrepresenting the true financial difficulties he has brought upon the NHS," he said.
"The prime minister must come to the House of Commons at the earliest opportunity and correct the record. He has been found out.
"He has cut the NHS budget for two years running and he owes it to patients and NHS staff to be honest about that."
A Department for Health spokesman said: "The 2010-11 year should not be used a baseline for NHS spending because the budget and spending plans were set in place by the previous government.
"For the first year of this government's spending review, as Andrew Dilnot acknowledges, NHS spending increased in real terms compared to the previous year by 0.1%.
"The NHS budget will continue to increase in real terms during every year of the current spending review settlement."