But private companies defended the proposal saying it would allow charities to take a bigger role in delivering NHS services.
The any qualified provider policy will see NHS and private providers compete for contracts based on quality and cost. The government says it will improve patient choice.
The NHS Future Forum opted to retain the policy in its recommendations on the Health Bill last month.
Karen Jennings, head of health at Unison, debated the plans during a lively exchange at the UK Faculty of Public Health's annual conference in Birmingham on Monday.
Ms Jennings told attendees: 'We are moving towards wholesale competition in the NHS.'
If the any qualified provider policy is adopted, 'we will have an NHS based on pure, unfettered market', she warned.
She claimed plans to increase patient choice could actually raise NHS costs if they opt for more expensive options and was 'perverse' at a time of stringent cuts.
The choice agenda is a first step towards co-payments for some care, she added, which will eventually lead to a US-style health insurance model.
'This is the road to ruin for the NHS,' she said.
The NHS Future Forum had heard concerns about the policy during its examination of the Health Bill.
But its final report, published in June, chose to leave the policy intact.
It said charities, social enterprises and independent providers are included in the definition of any qualified provider, and these currently offer 'excellent services that people choose to use'.
The report concluded: 'There is a wealth of talent and untapped resource in our country's third sector which can benefit the NHS, so there is a good argument for greater commissioning from alternative providers of care where appropriate.
'On this basis there is no strong case for changing the policy.'
The view was shared by Mark Britnell, head of health for advisory company KPMG, who insisted that the policy would give charities who provide NHS services already a greater role in future.
He cited work by the charity Whizz-Kidz to provide disabled children with wheelchairs. 'Should we stop it?' he asked the conference, rhetorically.
Speaking about the any qualified provider policy, Mr Britnell said a recent poll suggested 74% of people in England do not care who runs NHS services so long as they meet quality standards.
Mr Britnell said the NHS could survive the tough economic challenges it faces 'so long as it continues to evolve'.
He added that the 'game changer' was the recent Dilnot Commission report into the funding of care and support in the NHS and that the Health Bill will be seen as 'largely incidental' in 10-20 years time.