All fitness-to-practise hearings that start on or after 31 May where the facts are in dispute will be judged on the civil standard.
GPs with a hearing pending are due to hear from the GMC this week.
Doctors will be judged by fitness-to-practise panellists 'on the balance of probabilities' instead of according to the criminal standard that requires proof 'beyond all reasonable doubt'.
The GMC said around 100 doctors are currently in the queue for a hearing.
The switch, which has been almost unanimously condemned by the rest of the profession, is expected to bring a flood of appeals.
Dr Hugh Stewart, head of case decisions at the Medical Defence Union (MDU), said his organisation was 'very disappointed' that the GMC had ignored its concerns.
He said the lack of transitional arrangements was 'grossly unfair' and that 'a doctor might have been preparing his defence on the basis of the criminal standard and now finds due to timing that the civil standard will apply.'
Dr Stewart queried how fitness-to-practise panellists could know to what standard they were to establish the facts of a case against.
'The White Paper said that where a doctor's livelihood was at stake the civil standard would be virtually indistinguishable from the criminal standard. How can the panel know to what standard to find the facts first without knowing what the sanction is?'
He said multiple allegations cases could require different standards for each allegation.
The GMC insists that no more doctors will be erased from the register as a result of the move to the civil standard of proof.
Proof standard change
31 May 2008: All fitness-to-practise hearings that start on or after this date where the facts are in dispute to be judged on the civil standard.
This is on the balance or probabilities rather than beyond all reasonable doubt (criminal standard).
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