The union says implementation of plans set out in the health White Paper Liberating the NHS must stop until the public has been properly consulted.
It has applied to the High Court for a judicial review, claiming Mr Lansley's 'refusal to consult' is unlawful.
The proposals represent ‘the most fundamental changes to the way the NHS operates since it was created,’ said the union.
Soon after the White Paper was published, NHS chief executive Sir David Nicholson wrote to NHS executives instructing them on how to start implementing aspects of the reforms.
Karen Jennings, Unison's head of health, said the DoH was refusing to accept its legal duty to consult the public.
'The White Paper contains sweeping changes to the NHS and how it should be run. The NHS constitution enshrines the principle that the public, staff and unions have an absolute right to be consulted, and that means not only on how the proposals are to be implemented, but also whether they should go ahead in the first place'
Unison claims the reforms will open the door to privatisation of the health service.
A DoH spokeswoman said Sir David's letter was encouraging PCTs to begin local consultations and 'first steps' that would not pre-empt the wider consultation.
'The government is engaging fully with the public, healthcare professionals, local authorities and unions on how its proposals will be implimented,' she said.