NHS bosses have been ordered to get to grips with spending on services provided for patients with rare conditions.
If NHS England does not control its spending on specialised services, then other services such as general practice could "lose out", the Government spending watchdog has said.
The National Audit Office (NAO) said spending on specialised services - which include specific mental health problems, rare cancers and other rare conditions - has increased at a much greater rate than other parts of the NHS.
The budget for specialised services increased from £13 billion to £14.6 billion between 2013/14 and 2015/16, an increase of 6.3% a year on average, compared to an increase of 3.5% year on average for the NHS budget as a whole, the NAO said.
It warned if NHS England is unable to keep its spending on these services within budget, this will affect its ability to resource other services such as primary care, non-specialised hospital and community services.
"Against a backdrop of increasing pressure on NHS finances, NHS England has not controlled the rising cost of specialised services," said NAO head Amyas Morse.
"If specialised services continue to swallow up an increasing proportion of the NHS budget, other services will lose out."
An NHS England spokesman said: "For the first time in three years, specialised commissioning has successfully balanced its budget this year - which is a major achievement - but the NAO rightly points to the pressure we're having to manage from rapidly rising demand for extremely expensive new treatments.
"But there is no free lunch here, so continuing to balance the books will continue to require difficult choices about investment priorities."