Five months into 2009, extra payments from retired doctors have already pushed the GMC's income up by £1.5m.
The fees windfall amounts to more than 4% of the GMC's annual £83.3m income.
The GMC scrapped exemption from its £410 annual registration fee for over-65s last year because it was breaching age discrimination laws.
It started charging around 8,900 doctors over 65 in January.
Dr Krishna Korlipara, a part-time GP over 65 and former member of the GMC, said it was ‘iniquitous' for a ‘premium fee' to be extracted from over-65s who are not practising.
He called on the GMC to look into a sliding scale based on income.
Doctors pay the GMC the full registration fee unless their income drops below £21,862. They are then entitled to a 50% discount.
A GMC spokesman said: ‘The additional income from doctors over 65 will be factored into the 2010 budget-setting process, and so it will be taken into account in determining future ARF (annual retention fee).'
- Should the GMC be charging over-65s?
- Read next week's GP dated 7 August for the full version of this story.
More news from 31 July