More than 2,000 doctors at 50 university hospitals in Japan were found to have worked without pay, with many lacking an employment contract or compensation insurance, the education ministry said Friday based on a recent survey.

Japanese university hospitals have a practice of not paying wages to doctors -- many of whom are graduate students -- who treat patients as part of their research or training.

But the widespread practice has forced such doctors to work part time at other institutions to make a living, often exhausting them to the point that they fall asleep while treating patients.

More unpaid doctors are expected to surface as reviews on the status of more than 1,300 doctors are still pending, and experts say the latest findings are only the tip of an iceberg as the survey methods vary between hospitals.

The universities are expected to pay the doctors who have gone unpaid and sign employment contracts with them following the ministry's instructions.

"The findings are very regrettable, and it is only natural to correct the practice of not rewarding doctors who should be paid," said education minister Masahiko Shibayama.

The ministry had previously surveyed graduate students on whether they had signed an employment contract with their university hospital, but the survey did not focus on unpaid doctors.

The latest survey, conducted between January and May targeting 31,801 doctors and dentists who worked at 108 university hospitals in September last year, found that 2,191 doctors, or 7 percent, went unpaid.

Of them, 751 doctors at 27 hospitals were found to have been unpaid without a rational reason, including some who worked four days a week despite their contracts stating they should only work twice a week. The hospitals will make retroactive payments for their services dating back two years.

The remaining 1,440 doctors at 35 hospitals were unpaid for a reason, but the hospitals have decided to pay their wages from now on, given the frequency and the extent of the services they are providing.

Among the surveyed doctors, 1,630 had not signed an employment contract without a rational reason and 1,705 were without industrial accident compensation insurance.

"University hospitals have taken advantage of our conscience and exploited us," said a doctor in his 30s in Tokyo who had been forced to work full time six days a week as a graduate student, despite his contract showing he could work slightly over 10 hours a month.

To pay his tuitions and make a living, he had to take up a part-time job that put him on night shifts for up to 15 days a month.

Looking back, he said, "I was constantly sleepy and exhausted." The doctor said he nearly fell asleep during an operation and while he was listening to a patient's heart.

"This is not us asking 'help us poor unpaid doctors.' The lives of patients are at a risk if our lack of sleep interferes with our work and if we collapse while conducting an operation," the doctor said.

Naoto Ueyama, who heads a labor union of doctors in Japan, said the latest figures do not reflect the actual number of unpaid doctors and called for them to be paid in addition to being given employment contracts.

"Even if the doctors are at hospitals for training or research, they obviously need to be paid as the doctors are treating patients as licensed medical practitioners and hospitals are gaining rewards through their services," Ueyama said.

"Unless there is an employment contract, their working hours are not controlled. Patients could also suffer the disadvantage of not knowing who will take responsibility in case of a medical accident," he added.