A survey of 840 GP trainees by the King's Fund think tank found that 27% of trainees intend to work full time in general practice one year after qualifying, down from 31% in 2016.
But the proportion planning to work full time 10 years after qualifying dropped to just 5% - half the proportion who planned to work full time at this stage in their careers in 2016.
The figures come as official NHS data confirmed that that the number of full-time equivalent (FTE) GPs dropped by 339 in past year - a fall of 1.2%.
The number of GPs in training hit a record 3,538 in 2019 - and political parties have promised further expansion in training places in the current election campaign. But the King's Fund poll shows the challenge faced by the NHS to translate this into an expansion of overall GP numbers.
Intensity of the working day and a desire to spend more time with family were two of the main reasons cited by GP trainees for wanting to work less than full time, the survey found.
Just 41% of GP trainees said they were considering GP partnership 10 years after qualifying, down from 45% in 2016, with respondents most worried about the financial implications of running a practice.
The proportion of GPs considering portfolio working one year after qualifying increased by 6%, with 24% planning to work this way, and 51% planning to do so five years after qualifying.
Despite desires for a more flexible career, the study found that the intensity of the working day remains the leading factor in trainees not wishing to undertake full-time GP work, with 69% of respondents citing this.
Around a quarter of respondents said that a lack of staff was to blame for the current intensity of work, with increasing patient complexity and an ageing population other reasons for trainees believing the future workload of GPs would be ‘unmanageable’.
One trainee said working as a full-time GP would put their health at risk: ‘Patient volume and work intensity makes a nine-session week look intolerable in terms of risk of burnout and maintaining a decent standard of care.'
Another trainee highlighted increasing expectations: ‘Decreasing threshold for patients to present to their doctor with symptoms or problems – partly driven by a "just in case" mindset and partly because of an over-expectation of what modern medicine can offer in terms of diagnostics or resolution.'
Senior fellow at the King’s Fund Beccy Baird, said: ‘Given it is likely that many new GPs will spend only part of their working week in general practice, promises of 5,000 or 6,000 more full-time equivalent GPs will mean finding many more individual GPs, potentially as much as half as many again.
‘There appears to be a political arms race for who can set the highest target for recruiting new GPs, but unless the next government addresses the unsustainable workloads that cause GPs to leave or reduce their hours, those aims may fast become broken promises.’