Junior doctors go to the High Court today in a bid to block the Government's decision to introduce a new contract.
Justice for Health, the campaign group formed by the medics, is accusing the 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 contend his decision to impose the contract lacks a sound or rational foundation.
The dispute over the contracts has already led to thousands of operations and appointments being cancelled since strikes started in January.
Until now, the longest strike has only last two days. But next month, when the new contract is due to be introduced, thousands of junior doctors are planning the first five-day walk-out.
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 group's founding members are all junior doctors - Dr Nadia Masood, Dr Ben White, Dr Fran Silman, Dr Amar Mashru and Dr Marie-Estella McVeigh.
Their solicitor Saimo Chahal, from law firm Bindmans, 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.
Their dispute with the Government led to the first full walkout strikes of their kind in British history.
Junior doctors rejected the latest contract offer put to a referendum by the British Medical Association (BMA) in June.
The Health Secretary decided to impose the contract, leaving junior doctors complaining that their concerns had been ignored.
Mr Hunt told Parliament that the NHS needed certainty, including in light of the UK's decision to leave the EU.