Lowering the standard of proof at General Medical Council (GMC) hearings will mean doctors are unfairly penalised, the British Medical Association (BMA) claims.
The GMC plans to change the standard of proof to the civil level which uses "the balance of probabilities" when ruling on disputed allegations against medical professionals.
The watchdog currently uses the criminal standard where any accusations must be proven "beyond all reasonable doubt".
It says it will still maintain this threshold in serious cases, but a flexible use of the civil standard for minor cases "is wholly consistent with protecting patients and the public interest and with being fair to doctors".
But a statement from the BMA said: "Patient safety is paramount, and the BMA will continue to work closely with the GMC to ensure that the extremely small number of doctors who are unsafe are dealt with swiftly and effectively.
"It is essential the public trust in the medical profession is maintained and preserved.
"However, it cannot be right and fair to take away someone's entire livelihood simply on the basis of the balance of probabilities.
"The BMA will oppose this in the strongest terms. The government should think again and maintain the criminal standard of proof where a decision is being made that would prevent a doctor from working."