Some qualified GPs cannot find enough work even at a time of high patient demand for appointments, according to a doctors' union.
The British Medical Association (BMA) says it has heard from locum doctors in England who are struggling to get shifts at practices.
Locums are used by practices to cover sickness or other absence among partners or salaried GPs.
The BMA says the situation is "ridiculous".
Prof Philip Banfield, chairman of the BMA Council, said: "How is it possible to have thousands of patients needing treatment and GPs available to give that care, but prevented from doing so by a system unable to pay them?"
Some doctors choose to become locums so they can have flexible working patterns, including those who want to do a full working week.
Dr Hina Siddiqi, from Manchester, started doing locum shifts in 2019 after being a partner for nine years.
She said she almost burned out, and becoming a locum helped control her workload.
But this year she says locum shifts are almost non-existent and instead she has taken a two-day-a-week contract at a practice, although she would like to work a four-day week.
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https://www.bbc.co.uk/news/articles/cx77zg1d4q7o