More than 200 consultants have signed a letter to NHS England saying they are "deeply concerned by the un-costed, un-evidenced" plans for a seven-day NHS.
The letter, organised by the Junior Doctors' Alliance, comes as separate research from the British Medical Association (BMA) found almost half of doctors are so disillusioned with the NHS they are hoping to move abroad.
In the letter, consultants said they were concerned by plans to create a "truly seven-day NHS" - a phrase used by Health Secretary Jeremy Hunt - "at a time when there is abundant evidence that we are struggling to provide a five-day NHS plus emergency cover 24/7".
It said: "The ambition to expand and improve services is laudable but without extra staff, funds and resources it is simply not possible and this is felt keenly by consultants, junior doctors and all other staff across the NHS."
The BMA research followed the same group of doctors for a decade after graduation and found that, at the nine-year point, 42% believed their current experience as a doctor was worse than they expected when they graduated.
The study among 430 doctors also found that 42% plan to practise overseas, a slight increase on previous years. One in 10 said they had applied for a certificate of current professional status from the General Medical Council (GMC) - a marker of expertise - with a view to working abroad.
Figures from the GMC show that 5,383 doctors have been issued with a certificate so far this year, compared to 8,625 for the whole of 2015 and 4,925 in 2014.
Obtaining a certificate does not mean the doctor has actually left the country.
The BMA found that an increasing number of doctors compared to previous years were considering moving abroad or leaving medicine, but fewer wished to change their speciality.
Some 16% had taken a break from medicine, up slightly on the previous year, while doctors said their biggest sources of stress were maintaining a work-life balance, the shortage of doctors and lots of paperwork.
The BMA said the past four surveys have seen a deterioration in perceptions of working atmosphere, working conditions and complexity of work.
The union is currently embroiled in dispute with the Government over a new contract being brought in for all doctors below consultant level from Wednesday.
It has called off strikes planned for the run-up to Christmas and is looking at other ways of opposing the deal.
Dr Ellen McCourt, chairwoman of the BMA's junior doctors' committee, said: "We have been saying for some time that morale amongst doctors is at an all-time low and these figures show, once again, that doctors are on a knife edge. They are reaching their limit, and if stretched any further, they will walk.
"Given the results of this study, it makes no sense for the Government to rush the implementation of the junior doctor contract, which will only make things worse.
"With the NHS facing unprecedented pressure, it is critical to focus on how to assure its long-term future. Junior doctors are central to this. If even a small number choose to vote with their feet, the future looks increasingly uncertain."