More than half of biomedical research in England is funded by the National Health Service, yet less than half of the papers published acknowledge this funding.
These are findings of the first comprehensive study of research activities within the English NHS published by the Wellcome Trust.
Sally Davies, regional director of research and development at the NHS Executive London and co-author of the report, said no one knew the full extent of NHS research.
The study looked at peer-reviewed journals between 1990 and 1997, attempting to map clinical research in the NHS in a transparent and accountable way.
"Clinical research is a priority area for the Wellcome Trust over the next five years," said Jonathan Grant, co-author of the report and head of the trust's policy unit. "We want our funding activities to complement those already taking place within the NHS."
The report found £3.5 billion is invested in medical research every year. NHS England supports more than 13,500 papers per year, while the Wellcome Trust supports about 1,000.