A limit to working hours could prevent junior doctors gaining all the skills needed to practise safely, according to a new poll.
New European laws come into force next year cutting the number of hours junior doctors work from the current maximum of 56 to a maximum of 48.
While many junior doctors do not want to work longer hours, a new survey published showed that many were concerned about the impact of the limit.
More than two in three (64%) junior doctors surveyed by the British Medical Association (BMA) believed compliance with the 48-hour working week would have a "negative overall effect" on their training.
When asked what they were worried about the most, 33% feared an impact on the quality of their training while a further 32% were concerned about the impact on their ability to learn the skills needed to practise safely.
Ram Moorthy, chairman of the BMA Junior Doctors Committee, said: "The 48-hour limit is coming and it will have a massive impact.
"Our training has to get far better if we're going to continue to produce the best quality of doctors.
"We need to look at the possibility of lengthening the amount of time it takes to qualify as a consultant."
Some also said they were worried the new limit will see them working "bizarre and complex" rotas for less pay.
More senior members of NHS staff have previously expressed concerns about the limit's impact on the ability to train doctors in all the skills they need.
