Health secretary Jeremy Hunt has no legal grounds on which to impose the new junior doctors contract, lawyers acting for a group of medics have claimed.
A "letter of action" was served to the Department of Health by the law firm Bindmans LLP questioning whether Hunt had undergone the necessary consultation with relevant parties in NHS trusts to be able to impose the controversial new contract.
The letter lists more than 80 ways in which lawyers consider the imposition of the contract to be legally flawed.
The firm was asked to begin investigating judicial review proceedings last month after a group of junior doctors raised over £100,000, using the crowdfunding site Court Justice, to cover legal fees.
Lawyers led by Bindmans partner Saimo Chahal QC say Hunt acted unreasonably when he ceased negotiations between the government and the British Medical Association (BMA), and decided to go ahead with the contract despite opposition from doctors.
They also considered whether his decision may not be legal on the basis that the changes to doctors' working hours and pay structure would not achieve Hunt's goal of a "seven-day" NHS.
Hunt has been repeatedly criticised for attributing data that shows patients admitted to hospital at the weekend are more likely to die in the following 30 days to junior doctors staffing.
Charlie Massey, director general for external relations at the Department of Health, also recently admitted the government did not know how much the proposed plans would cost.
"There is no separate pot set aside for something with the specific label of seven-day services," he told a Commons public accounts committee hearing in February.
While NHS boss Sir Bruce Keogh does support Hunt's call for a "seven-day NHS," he has described the imposition of the contract as "one of the saddest days I have seen in the NHS".
The Department of Health now has seven days to respond to the letter before the judicial review will be taken to the high court.
“We hope this legal challenge will hold the government to account for imposing a contract that threatens the future stability of the NHS,” said Francesca Silman, one of the doctors involved in the legal action.
When the full contract was finally made public last week, almost two months after Hunt announced his plans to impose it, the BMA announced that it would launch its own judicial review action.
"Imposing this contract will seriously undermine the ability of the NHS to recruit and retain junior doctors in areas of medicine with the most unsocial areas, where there are already staffing shortages," Dr Johann Malawana, chair of the BMA's junior doctors' committee, said.
"The government must listen to the chorus of concern coming from all quarters and reconsider this disastrous approach."
A Department of Health spokesperson called the action "totally unwarranted."
"If the BMA had agreed to negotiate on Saturday pay, as they promised to do through ACAS in November, we'd have a negotiated agreement by now," they added in a statement to BuzzFeed News.
On Wednesday, junior doctors will stage their fourth emergency-care-only strike this year, with a full 48-hour walkout planned for the end of the month.
"The government can avert this action by re-entering talks with the BMA and addressing rather than simply ignoring the outstanding issues and concerns junior doctors have," Malawana said.
"If it pushes ahead with plans to impose a contract that junior doctors have resoundingly rejected, we will be left with no option but to take this action."