Junior doctors in England have gone to the High Court to try to stop the government imposing a new contract.
The group Justice for Health, which is mounting the legal challenge, says the contract is "unsafe and unsustainable" and it accuses Health Secretary Jeremy Hunt of acting outside his powers.
The Department of Health says the case is without merit.
Ministers insist the new contract is needed to improve levels of medical cover in hospitals at weekends.
The hearing at the Royal Courts of Justice in London will run for two days.
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.
The new contract is due to be rolled out from October.
'Very high risk'
Dr White said it was "incredible" that junior doctors have had to take the government to court.
He said: "Jeremy Hunt's own civil servants in the Department of Health said there was a very high risk to trying to bring in seven-day services without the required investment and staffing.
"And actually you've got us, the doctors and the nurses on the front line, saying, 'This is not going to be safe.'
"So that forms a key part of one part of our legal argument. That there's no definition even of a seven-day service."
Dr Mashru said the legal challenge would look at three areas: the lawfulness of the imposition of the contract; whether Mr Hunt had properly informed the public and Parliament about the plans; and the evidence behind Mr Hunt's reforms.
Junior doctors had planned a series of five-day strikes in September, October, November and December in protest against the new contract.
They called off the September strike after senior colleagues said the action was disproportionate and risked patient safety.
Barrister Clive Sheldon, who led Mr Hunt's legal team, told Mr Justice Green in a written statement: "The claim is wholly without substance."
He said Mr Hunt had felt that a new contract should be introduced by the NHS.
He argued that Mr Hunt had not decided to "compel" NHS employers to use the new contract, but had decided to "approve" the new contract.