A private firm is to carry out surgical work at an NHS hospital in south west London, campaigners say.
The government said private firms play an important role in the NHS
The London Health Emergency group said it has obtained documents showing that Kingston Hospital could start the controversial plan next year.
It described the move as an "unprecedented experiment" which would take private firms deeper into the NHS.
But the Department of Health said the independent sector had an important role to play within the NHS.
Geoff Martin, head of campaigns at London Health Emergency, called for the intervention of Health Secretary Alan Johnson.
He said: "The plans by Kingston hospital to bring a commercial, profit driven company right into the heart of the NHS to run all routine surgery is unprecedented.
"It would give that company enormous leverage to launch a takeover bid to run the whole hospital.
"This is NHS privatisation on a scale we have never seen before."
But a Department of Health spokesman said: "The independent sector has played an important role in driving forward NHS reforms, helping to slash waiting times and increase patient choice."
He added: "Primary Care Trusts will take responsibility for securing the best possible healthcare for their patients by commissioning services from a range of providers, including the independent and voluntary sector."