A second legal challenge has been made over the government's decision to impose a new contract on junior doctors in England.
NHS staff campaign group Just Health started proceedings after raising £100,000 in four days online.
The move follows separate legal action made by the British Medical Association last week seeking a judicial review.
A Department of Health spokesman said legal action was "expensive" for all parties and "totally unwarranted".
The news comes before Wednesday's planned 48-hour strike affecting routine and non-urgent care is due to start.
The strike will be the fourth time doctors have taken industrial action in the long-running dispute.
Just Health alleges government's decision to impose the contract, affecting about 45,000 medics, did not allow for adequate consultation.
The group raised the £100,000 through a crowdfunding website which remains open to donations.
Dr Marie-Estella McVeigh, from Just Health, said: "We feel this contract imposition has been rushed through without appropriate consideration and due process.
"There is no evidence that it will deliver a safer system or better quality care for our patients; it will instead exacerbate the staffing crisis we are already struggling with across the NHS."
The BMA, in its legal claim, alleges that the government failed to "pay due regard" to the equalities impact of the new contracts and is calling for a judicial review.
Talks between the government and BMA broke down in January, prompting the government to announce the following month that it would be imposing the contract in the summer.
Ministers have said the changes, which will see doctors paid less for working weekends, are needed to improve care at weekends and create a "seven-day service". This is disputed by the BMA.
Currently, junior doctors are paid more for working unsocial hours at night or at the weekend.
But under the imposed new contracts, the Saturday day shift will be paid at a normal rate in return for a rise in basic pay.
Last month, the Department of Health described the forthcoming strike action by doctors as "desperate and irresponsible".