Junior doctors have been told to prepare for strike action as talks between the British Medical Association (BMA) and the Government have yet produce an agreement, according to reports.
Walkouts were suspended last month following five days of negotiations on medics' proposed new contracts, but talks appear to have again reached an impasse ahead of a Monday deadline.
The Guardian said Dr Johann Malawana, chairman of the BMA's junior doctors committee, told his members in an update that while "significant ground has been made ... there are still some areas of absolute disagreement".
He added: "It is therefore absolutely clear that should we not be able to reach an acceptable outcome by 4pm on 4 January, the BMA will need to commence serving notice as per its mandate, to the NHS, for industrial action the following week."
The BMA has maintained the right to hold its strikes before a deadline of January 13 should talks break down, while the Government could still impose the new contract in lieu of a settlement.
Danny Mortimer, the chief executive of NHS Employers, which is working with the Government, criticised the BMA over the warning.
He told the paper: "Sensitive negotiations are not a spectator sport and I still do not propose to provide a running commentary on their progress since the Acas agreement of 30 November. I am disappointed therefore that this BMA communication has unexpectedly appeared.
"Talks with the BMA about the proposed new contract for junior doctors have been adjourned until 4 and 5 January 2016, though work to refine details is continuing in the interim between members of our teams. We believed that we have seen progress and we will resume discussions in the new year."
A spokesman for the Department of Health said: "We have been making good progress with the BMA and look forward to further discussions in the new year.
"Talks are always better than strikes and we are prepared to talk about anything within the pay envelope as long as we improve patient safety by moving towards a seven-day NHS. Our proposals offer better basic pay and safety for junior doctors, with a shorter working week."
Under the proposed contract changes, junior doctors in England will get an increase in their basic pay, but will have to work more weekends while guaranteed pay rises linked to time in the job will be axed and bonus pay for working unsociable hours will be curbed.
Some 98% of junior doctors balloted by the BMA voted for industrial action.
Even though the strikes planned for this month were averted at the 11th hour, the threat caused mass disruption to the NHS as thousands of patients had operations or appointments cancelled.