There is a risk junior doctors will "speak with their feet" and leave the UK if the Government imposes a new contract on them, the British Medical Association (BMA) has warned.
The union said there had been an "outpouring of anger" over plans to impose the contract from next August.
The BMA has decided not to re-enter negotiations over the contract, although it will still "get back around the table" with NHS Employers regarding a new contract for consultants.
A ballot on industrial action over the issue has not been ruled out.
The contract will reclassify junior doctors' normal working week, known as "plain time", 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.
They say this amounts to pay cuts of up to 30%.
New figures from the General Medical Council (GMC) showed a rise last week in the number of doctors seeking a certificate of current professional status, which is needed to work abroad.
The certificates are valid for three months. Not all doctors who request a certificate leave the UK.
Between Wednesday September 16 and Friday September 18, the GMC received 1,644 requests for certificates of current professional status.
It said it usually receives an average of between 20 and 25 requests a day.
To date, the GMC has issued 4,500 certificates so far this year, compared to 4,925 in the whole of last year.
Dr Kitty Mohan, co-chair of the BMA's junior doctors' committee, said: "These figures should serve as a serious wake up call to the Government.
"There has been an outpouring of anger over plans to impose a new contact and there is a real risk that junior doctors will speak with their feet.
"To lose a large swath of doctors in the early stages of their careers would be a disaster for the NHS."
A spokeswoman for the Department of Health said: "These figures are not emigration data and do not reflect the number of doctors planning to leaving the UK.
"We suspect this sudden spike in CCPS applications is prompted by the doctors' trade union, which is deliberately misrepresenting our contract proposals.
"NHS staff are our greatest asset and in light of the BMA's disappointing decision to decide against re-entering contract negotiations, NHS Employers will continue to develop a contract that is good for junior doctors and patients, increases basic pay and rewards those who work across all clinical specialities."
The Department of Health said the contract enhances training opportunities for junior doctors, gives a significant increase in basic salary and offers "proportionate" payment for extra hours worked.