More than 80% of hospital doctors and 67% of GPs also pointed to variations in care, saying there are certain doctors they would not want to treat their friends and family.
The survey by doctors.net.uk was carried out among more than 4600 hospital doctors and a further 1000 GPs. It was released ahead of the anniversary on Monday of serial killer Harold Shipman's death.
Doctors were asked whether revalidation - the process of appraising doctors - would help identify and deal with those who are unfit to practise. Some 53% of hospitals disagreed it would (33% disagreed and a further 20% strongly disagreed). Meanwhile, 22% agreed revalidation would help identify and tackle problem doctors, while the rest neither agreed or disagreed or did not know.
Among GPs, 60% disagreed the plans for revalidation would help to identify and deal with doctors who are not fit to practise (32% disagreed and a further 28% strongly disagreed).
Some 86% of hospital doctors also agreed there are variations in care and that "there are certain doctors that I would not want to treat friends and family".
Dr Tim Ringrose, chief executive of doctors.net.uk, said: "GPs and hospital doctors seek to uphold the highest possible standards in care, and their willingness to be totally frank about variations in quality demonstrates how keen they are to see continual monitoring and improvement.
"However, while revalidation should help to address such concerns, there is widespread scepticism about its effectiveness."
Shipman, who was 57 when he died, was jailed for life in January 2000 for murdering 15 patients.
Shipman's conviction led to scrutiny regarding the professional regulation of doctors. A new system of revalidation is now being implemented.