UnitingCare won a competitive tender to run the contract to supply older people and adult community healthcare in Cambridgeshire. It began on 1 April.
Jo Rust from the union Unison said "it's evident that they can't make financial cost savings" promised.
Patients have been told services will not be disrupted.
Older people's services have been transferred back to Cambridgeshire and Peterborough Clinical Commissioning Group (CPCCG).
Dr Neil Modha, its chief clinical officer, said "both parties" had decided to end the contract because "the current arrangement is no longer financially sustainable".
He added "we all wish to keep this model of integrated service delivery" and "services will continue and not be disrupted".
UnitingCare declined to be interviewed.
It is a consortium of Cambridgeshire and Peterborough NHS Foundation Trust (CPFT) with Cambridge University Hospitals NHS Foundation Trust.
The company was selected to run the service by CPCCG after a lengthy procurement process and from a shortlist of three organisations.
What does the £800m five-year contract cover?
Ms Rust said she was "totally shocked" at news the contract had come to an end.
"It cost over £1m to commission the UnitingCare partnership, to put it out to tender - and it was a waste of money," she said.
"But it will be business as usual as these services are vital to the health economy of Cambridgeshire and Peterborough, despite the uncertainty."
A five-year £800m NHS out-sourcing contract has ended after eight months because the company running it said it was not "financially sustainable".