Junior doctors will go to the High Court on Monday in a bid to block the Government's decision to impose a new contract on them.
Justice for Health, the campaign group formed by the medics, is accusing Health Secretary Jeremy Hunt of seeking to impose "unsafe and unsustainable" terms and conditions as he presses ahead with controversial plans for seven-day NHS services in England.
Lawyers for the doctors are expected to argue in a two-day hearing in London that Mr Hunt has not only acted unlawfully but "misled Parliament".
The medics are arguing that, although Mr Hunt is entitled to "recommend" a new contract, he is attempting to go significantly further even though he has no power to decide the terms and conditions under which the NHS and other bodies should employ junior doctors.
The doctors also believe that Mr Hunt's decision to impose the contract lacks a sound or rational foundation.
Junior doctors have held several strikes since January over the proposed changes, leading to thousands of operations and appointments being cancelled.
Until now, the longest strike has only lasted two days, but from next month medics have planned a series of five-day walk outs.
Saimo Chahal, solicitor for the doctors, described them as "incredibly brave" in seeking to hold Mr Hunt to account.
Ms Chahal said: "They are fighting not about conditions of employment but about issues which lie at the heart and soul of the NHS."
Dr Silman said: "We have spent the last year trying to explain to Jeremy Hunt why the contract is flawed, and why it is irrational to continue with imposition, given the current staffing and funding crisis in the NHS.
"Mr Hunt has ignored doctors' concerns, and so we are forced to turn to the courts."
Thousands of members of the public have offered the doctors money via crowdfunding website Crowd Justice to bring their case.