The West Midlands GP was speaking to a London conference last month about recertification.
He said research by the GMC into performance had looked at inner-city single-handed GPs.
'Given the situation that they work in, the quality of care is actually fantastic,' he said.
'However, if you look at a group practice in a leafy suburb you have a GP hidden by other doctors and the collusion of those doctors.'
He suggested both a practice accreditation system and being able to focus on individuals.
Professor Field is a member of the regulation White Paper working group looking at revalidation which is chaired by Sir Liam Donaldson, the CMO for England.
Change is expected in the 12 months after spring 2008.
Professor Field was speaking at a conference organised by the British Association of Medical Managers entitled 'A Practical Guide to Appraisal and Revalidation in Primary Care for GPs and PCT Managers'.
Professor Field added: 'GPs are doing a fantastic job providing services in the UK often in very difficult situations.
'However, we do have a few GPs who are not up to scratch and we need to ensure that we can identify those and support them. 'If we can't support them, then we need to suggest that they go and do something else.'
Professor Field stressed the importance of sensitivity when dealing with soft information.
'The Shipmans of this world would not have been identified by any systems that have been put forward so far,' he added.
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