Part-time working for GPs is becoming commonplace, regulators say, raising concerns about the government's drive to recruit extra doctors.
Some 45% of GPs are working less than full-time, with a third cutting their hours in the past year, a General Medical Council survey indicated.
The poll also suggested more intended to follow suit amid rising workloads.
The regulator said it was essential to retain more full-time GPs if numbers were going to rise.
'I do full-time hours on a three-day week'
Dr Lucinda McWhor is one of many doctors who is part-time.
She works three days, but still clocks up 35 to 40 hours a week – the equivalent of a full-time role.
"All GPs I know work around two to four hours a day unpaid.
"This is why they are part-time, they are doing full-time hours but getting paid part-time rates.
"Because we are a predominantly female workforce this has been allowed to happen.
"Women have often worked part time in general practice and we tend to be ready to do more without the pay.
"At this current level of workload it is simply not safe to have GPs working five days per week.
"The days are non-stop and we need constant mental alertness."
The findings – part of an annual report into the entire medical workforce – come after the government promised to recruit extra GPs, as part of their election campaign.
It has set a target to recruit 6,000 extra by 2025 in England.
But GMC chief executive Charlie Massey said while more GPs were coming through training, it was not enough to keep up with demand.
"The clinical work of GPs is changing," he said.
"They're seeing more patients, many with complex needs and some who have high expectations of what primary care can do for them.
"To break this cycle of workforce shortages we need a clear plan, in all four countries of the UK, for a sustainable increase in the number of GPs."
In 2015, the government in England set a target of increasing the number of full-time equivalents by 5,000 by 2020.
But latest figures show the numbers have hardly changed.
Number of full-time equivalent GPs
There was an increase of 272 between 2015-2018. The target is 5,000.
The survey, of nearly 3,900 doctors – more than 1,0Read More – Source