Practices that marginalise locums expose patients to risk, Dr Fieldhouse believes.
Increasingly locums 'who are unfamiliar with local systems and processes' are being hired, he says.
Writing in the Medical Protection Society publication Your Practice, published today, Dr Fieldhouse says locums are bedevilled by a lack of national recognition and political power.
'Only Wales and Scotland can tell you with certainty how many locums are on their performers' lists,' he writes. England does not publish numbers of locums.
Dr Fieldhouse calls for 'significant resources' to be devoted to supporting locums 'to work within managed organisations'.
'We need to fight the system of enforced underperformance that permeates the locum profession,' he adds.
Dr Judith Harvey, who recently retired as a locum, writes that practices must induct locums carefully. Logging them in as 'Dr Locum' is 'unacceptable'. Locums need a smartcard and an up-to-date locum induction pack, she added.
One GP in four now works as a locum, an NASGP survey found last year.
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