They should also warn patients that data may be used for financial and administrative purposes such as the QOF.
Guidance published this week says doctors may disclose patient data for such purposes, but anonymise it if possible.
If that is not possible, they should seek patient consent, and provide posters or leaflets explaining how to object.
GMC policy adviser Michael Keegan said policy had not changed. The advice was issued to address large numbers of queries the GMC receives on confidentiality and QOF.
Many PCTs like to check the accuracy of QOF data against practice records, Mr Keegan said. But some practices still tell the GMC that their IT systems 'don't allow names to be removed without receptionists whiting out names'.
'The important thing is that patients are informed and realise they have a right to object,' he said.
The guidance also says doctors must inform the DVLA when patients continue to drive despite being unfit to do so.
It reiterated warnings to doctors against responding to criticism in the press, noting that: 'Although this can be frustrating or distressing, it does not relieve you of your duty to respect your patient's confidentiality.'