Under-pressure NHS services in England are spending over £1bn a year buying care from outside the NHS because they are unable to keep up with demand.
The bill is being racked up by hospitals, ambulances and mental health trusts, data obtained by the BBC shows.
NHS managers said money was being wasted as often it was done at the last minute, and led to the NHS over-paying.
Sending patients to private clinics for care like hip and knee surgery is thought to be the most common purchase.
Phillippa Hentsch, of NHS Providers, which represents trusts, said these decisions were often made as a "last resort" where the only alternative would be to cancel.
"Hospitals have to hand over the patients because they have simply not got the beds, staff or theatres free to see them due to the pressures on the emergency side.
"The best interests of the patients are what are paramount. But it is valuable income that is lost to those hospitals. It seems such a waste" she said.
"In some cases hospitals are over-paying for these treatments and tests. It is another sign that things are not working properly."
'A time of immense pressure'
The figures have been provided to the BBC by NHS Improvement, the financial regulator for the health service.
They show that in each of the last two years just over £1bn has been spent by NHS trusts on buying health care from non-NHS bodies.
Figures are not available for earlier years.
As well as non-emergency operations, the spending is also going on hospitals buying places in care homes to get elderly patients off wards.
Ambulances sometimes have to use private crews to transport the less serious patients, and mental health bosses have paid for beds in private hospitals because they have run out of space.
Charities are also used to provide support in areas such as cancer and palliative care.
David Hare, chief executive of the NHS Partners Network, which represents private health care firms, said there were "considerable benefits".
He said the flexibility of being able to buy in support could help reduce pressure and improve care.
"It is right that NHS trusts are able to make local decisions about how best to spend precious NHS resources. At a time of immense pressure it makes no sense to leave available capacity from the private and voluntary sectors 'on the shelf'."
A Department of Health and Social Care spokesman said that despite the sums involved the amount spent was still "low" given the overall size of the budget - for NHS trusts it exceeds £70bn.
"We remain clear that the NHS will remain free at the point of use both now and in the future," he added.