Trusts are still unclear about their role in doctors' revalidation, 10 years after the idea was first mooted.
A long-awaited government report published last week has not fully set out how new "responsible officers" will work with local outposts of the General Medical Council or how the policy will be costed.
Under the plans, the responsible officer will have to assure the GMC that the doctors employed in a particular trust or geographical area are fit to practise.
While the report says this officer will usually be a medical director, it is unclear what happens in primary care trusts that do not have medical directors.
It has not set out accountability structures for the proposed "GMC affiliates", who will provide advice to trusts and PCTs, and how any conflicts of interest will be resolved.
NHS Employers head of programmes David Grantham said: "There are still a lot of questions to be answered. You can envisage a situation in an acute trust where the medical director is the responsible officer and is too close to the affiliate, perhaps carrying out their appraisals."
The report follows last February's white paper, Trust Assurance and Safety - the regulation of health professionals in the 21st century. Plans for revalidation go back further, stemming from the 2005 Shipman inquiry, which called for a model proposed by the GMC in 1998 to be strengthened.
Mr Grantham said it was important for the Department of Health to set out how the two new roles will differ and who would be allowed to take on the responsible officer role; for example, whether it would have to be someone employed by the PCT.
Royal College of GPs chair Steve Field urged PCTs to prepare themselves for the changes. "Responsible officers in PCTs will have a really massive role. PCTs really must have a medical director who can engage with local GPs," he said.
Pilots on GMC affiliates and their relationship with responsible officers will start later this year. The full system will not be in place until after 2009-10, so lessons from early adopter sites can be incorporated.