Junior doctors have demanded "concrete assurances" from the Health Secretary, saying the "threat" that a contract will be imposed upon them must be removed.
In response to a letter from Jeremy Hunt last week, the head of the junior doctors' committee at the British Medical Association (BMA) indicated doctors will not budge on their demands for Saturdays to be paid at a higher rate.
Under the current plans, the contract will reclassify doctors' normal working week to include Saturdays and up to 10pm every night of the week except Sunday.
Medics argue they will lose out financially as evenings and Saturdays will be paid at the standard rate rather than a higher rate.
Mr Hunt said last week he would be pleased to discuss how far a normal working week would extend on Saturdays.
He guaranteed the contract would not impose longer hours on doctors and that the "great majority" of doctors would not lose out financially.
But in his reply, Dr Johann Malawana said there was a need for stronger words from Mr Hunt.
He said: "Our first concrete assurance centres on the withdrawal of the threat to impose a new contract.
"Your letter makes no mention of this, so we continue to seek urgent clarification from you as to whether the government still intends to persist with its timetable for the imposition of a new contract."
He said it remained the BMA's position that "work on Saturdays and late evenings cannot be considered as exactly the same as daytime on a weekday".
Furthermore, Dr Malawana said Mr Hunt's assurance that most junior doctors would be at least "as well paid" as they are now means, by definition, "that some junior doctors will receive a pay cut".
He went on: "I have spoken to many junior doctors across the country, who are telling me that they are becoming increasingly disillusioned and I believe this may have an impact on the career choices they are making in the UK and globally.
"Our concern is that this puts in jeopardy your manifesto commitment to deliver a seven-day NHS."
The letter from the BMA means strike action is still on the cards and formal negotiations with NHS Employers will not happen for now. Thousands of junior doctors are expected to congregate in central London on Saturday in protest at the planned contract.