A judicial review by a group of campaigners challenging government health policy in England gets under way at the High Court later.
The group, which was backed by Prof Stephen Hawking before his death, is fighting the creation of 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 – or integrated care organisations as they are sometimes known – which will hold contracts to provide services.
'Irresponsible scaremongering'
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, which includes leading members of campaign groups Doctors for the NHS and Keep Our NHS Public, say an act of Parliament would be needed for the changes.
But the Department of Health and Social Care has said the claims are irresponsible scaremongering.
The department argues they are simply about making care more joined-up.
Ten areas have already started piloting accountable care systems, including Greater Manchester and Surrey, which are part of the government's devolution programme.
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