Junior doctors and medical students chose to reject the proposed contract because it did not address some key concerns about unsafe working hours, one junior doctor has said.
GP trainee Dr Francesca Silman, from London, said she voted against the new deal because of worries over working hours.
She said the new deal also did not do enough to ensure staff are retained or encouraged to go into difficult specialties.
Dr Silman, who is part of the campaign group Justice for Health, said: "We are currently working in an underfunded, understaffed NHS, where unnecessary risks are taken on a daily basis due to lack of staff to provide the care our patients need.
"We need a contract that ensures staff are retained, and encouraged to enter some of the most difficult specialties such as emergency medicine.
"We need a contract that ensures that doctors are not working unsafe hours.
"I did not see these concerns addressed by the contract, whose premise is to stretch a workforce even further to provide the Government's manifesto of a seven-day NHS.
"For this reason, I felt there was no alternative other than to reject the contract.
"This is a pivotal moment for the NHS, and I strongly feel that if doctors do not fight for a sustainable workforce, the whole system is on the brink of collapse."
She added: "Moving forward, the Government must now acknowledge that it is not possible to provide a seven-day NHS without extra staff and funding.
"That it is not possible to create a contract that doctors feel is safe, by stretching the current workforce.
"Once this is acknowledged, then perhaps some solutions can be found, and a further contract can then be negotiated that is acceptable to everyone."
Meanwhile Dr Mark McInerney, who will be starting emergency medicine specialist training in London in three weeks' time, said he rejected the terms of the new contract because of a number of concerns.
He said he had concerns over whistleblowing protection and a new "guardian" role proposed under the deal.
And the new contract offered a "very complex" pay scale, he said.
Dr McInerney added: "I am so disappointed that the Government made me strike on this issue, or at least backed me into that corner.
"I'd be prepared to strike again, as I want fairer conditions for my female colleagues who often work less than full time compared to males. I also want 100% whistleblowing protection.
"The current significant under-staffing in the NHS means that many of the possibly good elements of the contract are completely undermined and mean nothing."