A ballot is due to close that could lead to thousands of junior doctors going on an "all-out" strike for the first time ever.
The British Medical Association (BMA) has proposed three days of strikes if doctors vote for industrial action in a bitter row with the Government.
The union, which has sent ballot papers to more than 30,000 members, is expected to release the results in the next few days.
If there is a Yes vote, junior doctors will only provide emergency care for 24 hours from 8am on December 1, followed by full walkouts from 8am to 5pm on December 8 and 16.
Health Secretary Jeremy Hunt has said the strikes - the latest stage of a fierce dispute over pay and conditions regarding a new contract for doctors in England - were "totally unwarranted" and would harm patients.
The union said it released advance notice of possible strike dates to enable the health service to prepare for the action.
BMA council chairman Mark Porter said the union "genuinely" wants to minimise any disruption to other NHS staff "and, above all, to patients".
The union has refused to get back round the negotiating table with the Government in the row over a new contract, which is set to be imposed from next summer on doctors working up to consultant level.
Mr Hunt had made a bid to avert strikes with a fresh deal, including an 11% rise in basic pay and overtime pay after 7pm on Saturday evenings, a concession on the previous 10pm.
Flexible pay premiums would be applied to more specialities than just general practice and A&E care, with acute medical ward staff and psychiatrists benefiting, he said.
He argued that just 1% of doctors would lose pay because of the deal and those were limited to doctors working too many hours already.
He said maximum working hours per week would fall from 91 to 72 under the new deal.
Johann Malawana, the BMA's junior doctor committee chairman, has said the increase in basic pay was misleading as it would be "offset by changes to pay for unsocial hours - devaluing the vital work junior doctors do at evenings and weekends".
The BMA has told the Government it wants to work with it to agree a new contract, but has said it needs the threat of the contract being imposed to be lifted.
It is also calling for proper recognition of unsocial hours as premium time, no disadvantage for those working unsocial hours compared with the current system, and no disadvantage for those working less than full-time and taking parental leave compared with the current system.