One in three NHS funded knee replacements carried out privately
The number of operations transferred to the private sector has risen steadily since 2012.
Nearly a third of NHS-funded knee replacements are carried out by private healthcare providers.
New figures also reveal one in five hip replacements are transferred to the independent sector.
The Royal College of Surgeons (RCS) said soaring demand and long waiting lists are forcing the NHS to turn to others for help.
The RCS warned this is having a negative impact on young surgeons, who are unable to access training opportunities in the independent sector and are struggling to carry out enough operations to meet training requirements.
The data, published in response to a parliamentary question by Labour peer Lord Philip Hunt, shows 28,700 NHS-funded knee replacements were carried out by independent providers in 2016/17.
This represents 29.4% of NHS knee replacements that year, compared with 20.1% in 2012/13 when there were 16,688.
Meanwhile, 22,872 NHS hip replacements were carried out by the independent sector in 2016/17, accounting for 19.7% of the total.
This was an increase from 2012/13, when there were 14,427 undertaken by private providers - 13.7% of the total.
Professor Derek Alderson, president of the RCS, said: "Faced with growing demand for services and unacceptably long waiting lists, the NHS is increasingly looking to the independent sector to relieve some of the pressure.
"This is especially true in hospitals where there aren't enough free beds to allow planned surgery to go ahead.''
While private hospitals provide "much-needed extra capacity'' and help patients receive treatment more quickly, he warned the trend risks drawing resources and staff away from the NHS.
He added: "Ultimately, we would like to see the NHS grow to meet patient demand.
"The Government's long-term plan for the NHS must focus on creating the capacity that is needed, including beds, whether that be by building new wards, reopening wards that have been mothballed or ensuring there are enough staff to work on wards.''
Jonathan Ashworth, shadow health and social care secretary, said: "Yet again this is more evidence that years of cuts to bed numbers, whilst forcing on hospitals the tightest financial squeeze in history, has wider consequences that undermine the fabric of our NHS.
"Ballooning waiting lists mean patients left in agony longer, and hospitals bosses forced to use the private sector to cope with demands.
"This isn't sustainable and taxpayers shouldn't be paying for a private sector bonanza like this. Tory ministers can't keep burying their heads in the sand. They need to get a grip urgently of growing waiting lists.''
A spokeswoman for Health Education England (HEE) said: "Health Education England is working with the Royal College of Surgeons to promote excellent training opportunities in order to provide high quality surgical care.
"HEE will facilitate training in non-NHS providers if the standards for training are met including the appropriate supervision from consultants is in place.''
A Department of Health and Social Care spokesman said: "Two million more operations were carried out last year compared to 2010 and for decades the NHS has used the independent sector to carry out some operations like knee and hip replacements where this is in the best interest of patients.
"As part of our long-term plan for the NHS, we will increase funding by an average 3.4% per year, meaning that by 2023/24 it will receive an extra #56 million every day than it currently does."