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.
“My skills and experience should be utilised by the profession and patients, and not wasted by my staying at home for five days of the week. I am not unemployed but underemployed," she said.
The BMA carried out a self-selecting survey which had more than 1,800 responses from locum GP members.
More than 1,500 said they were struggling to find work or had started looking outside the NHS. One, according to the union, has taken work as a freelance GCSE tutor because she could not find enough shifts.
The BMA has argued that the government funding of GP practices in England rose by just 1.9% above inflation in this financial year, and that some are cutting back on posts and funding for temporary staff.
Dr Mark Steggles, BMA sessional GP committee chairman, said: "It leaves us in the ridiculous situation where so many patients are being denied the chance to see a GP, even though we have GPs wanting to work and care for them."
The Royal College of GPs (RCGP) also says it has heard from GPs about difficulties finding vacancies.
Prof Kamila Hawthorne, who chairs the RCGP, said: "It is staggering to see our members say they are struggling to find jobs when general practice is overworked, overwhelmed and in desperate need of more GPs."
However, some argue there are wider problems to be addressed.
Dr Richard Fieldhouse is chairman of the National Association of Sessional GPs, which runs a booking platform connecting practices and locum doctors. He said there was the same amount of work, but it was "more thinly spread" because more medics wanted locum work.
But he acknowledged there was a "disconnect" in the way practices need doctors to cover their workload and that the systems for finding them were "out of date". He added that sometimes there were not enough consulting rooms in practices to fit in enough GPs.
Others with knowledge of the medical profession have suggested that locum rates were becoming too expensive, so practices had to cut back on their use. In effect they were pricing themselves out of the market. They note though that there are regional variations with demand for locums.
The Liberal Democrats said the blame for the situation lay squarely with the Conservatives and they had failed to recruit enough GPs.
The Conservatives said 50 million extra GP appointments had been delivered in England since 2019. Labour’s manifesto said, if elected, thousands more doctors in general practice would be trained. Reform UK were also approached for a comment by the BBC.