The number of NHS-funded hip operations carried out at private hospitals has increased tenfold in a decade, figures show.
Researchers from Newcastle University and Queen Mary University of London found the NHS outsourced around one in five (14,326) of its operations in 2012/13, up from around one in 50 (1,050) in 2004/5.
In November, official figures revealed the number of outsourced hip replacements had climbed even further to around one in three.
The researchers claimed that private hospitals were less likely to operate on older patients from more deprived areas.
They found the number of private operations on NHS patients in the most affluent areas increased by 288% between 2007/8 and 2012/13. The increase was 186% among patients from poorer backgrounds.
Professor Allyson Pollock, director of the institute of health and society at Newcastle University, said warned that if the trends continue widening inequalities are likely.
Professor Neil Mortensen, vice president of the Royal College of Surgeons, told the Daily Mail the NHS was looking to the private sector to relieve some of the pressure of unacceptably long waiting lists.
“While we recognise the need to use the independent sector so that patients can be treated more quickly in the short term, it risks drawing resources, for example nursing staff, away from the NHS in the long term,” he added.