Cancer care across Staffordshire could be privatised as part of a £1.2bn contract, a trade union has warned.
In what is believed to be the biggest outsourcing deal in NHS history, four clinical commissioning groups (CCGs) are jointly tendering for a body to integrate care across the county.
The Unison trade union said it could mean the privatisation of care and described it as a "huge gamble".
The CCGs said patients' treatment would be unaffected.
Andrew Donald, chief officer for Stafford and Surrounds and Cannock Chase CCGs, said no decision had yet been made on a provider.
"It could be an NHS body, it could be a voluntary body, it could be a private body or it could be all of them working together," he said.
The successful bidder will be asked to co-ordinate cancer care throughout Staffordshire - from diagnosis through to treatment and end-of-life care.
It will cover the four CCGs that commission services across the county - Stafford and Surrounds, Cannock Chase, Stoke-on-Trent and North Staffordshire.
Unison, however, said the deal could lead to the privatisation of care, "affecting the lives of many thousands of patients and staff".
Christina McAnea, Unison's head of health, said: "This is by far the biggest procurement process in the NHS and is a dangerous experiment.
"We are talking about £1bn of taxpayers' money and contracts lasting 10 years in vital cancer services and end-of-life care.
"This is much bigger than just asking private companies to provide a service, this is asking them to design the whole system. With profit as the main driving force, how can it not lead to problems?"
The union also criticised the transparency of the tendering process.
Mr Donald, however, said the bidding process was intended to lead to better value for money.
"The NHS in lots of areas is inefficient and we've got to make the taxpayers' pound go further," he said.
"One of the challenges is that we've got lots of organisations providing healthcare to the same patients and what really we struggle with is how those organisations work together.
"What we feel is if we can get an organisation to help to integrate those services we'd get a better deal for patients and carers."
A spokesperson for the Department of Health said: "The key thing is that patients get the best possible care, free at the point of use, no matter who provides it."