A group of campaigners, including Prof Stephen Hawking, has been given permission to challenge a government health policy in the High Court.
They will pursue a judicial review against Health Secretary Jeremy Hunt and NHS England over plans to create accountable care organisations (ACOs).
These are to act as partnership bodies incorporating hospitals, community services and councils.
Campaigners say it risks privatisation, but this is denied by ministers.
NHS England wants hospitals and other trusts to work closely with GPs and social care services to look after more patients in their communities rather than in hospital.
In some areas, these groups are developing into ACOs, which will hold contracts to provide services.
Critics argue that this could pave the way for privatisation of parts of the NHS and that Parliament has not legislated to allow the process to happen.
The group bringing the case to court says an act of Parliament would be needed for the changes.
Former Halifax and Huddersfield consultant eye surgeon Dr Colin Hutchinson, who chairs Doctors for the NHS, said: "These radical changes will eventually affect everybody in England.
"There needs to be a sound legal basis before 10-year contracts worth billions of pounds are outsourced to these new organisations."
Legal costs will not be capped if the case is lost, and the claimants are said to be considering their next steps.
The Department of Health and Social Care said the claims would be resisted and it was irresponsible scaremongering to say accountable care organisations were supporting privatisation.
A spokesman said: "The NHS will remain a taxpayer-funded system free at the point of use; ACOs are simply about making care more joined-up between different health and care organisations.
"Our consultation on changes to support ACOs is entirely appropriate and lawful.
"We believe it is right that local NHS leaders and clinicians have the autonomy to decide the best solutions to improve care for the patients they know best - and any significant local changes are always subject to public consultation and due legal process."