The proportion of GPs working a full week in Scotland has decreased at a period of increasing demand on their services, according to findings branded "very concerning" by doctors' leaders.
The 2017 primary care workforce survey found a "consistent decline" in the estimated number of whole time equivalent (WTE) GPs in four years, down by 160, or more than 4%, since 2013.
Almost a quarter of the GP practices responding to the survey also reported current GP vacancies, taking the vacancy rate from 1.7% in 2013 to 5.6% in 2017, the study said.
The Royal College of General Practitioners (RCGP) in Scotland said the findings highlight the need for strong "action to tackle the GP shortage sooner rather than later".
Ministers said primary care workforce levels are at a record high and insisted the new GP contract will ensure doctors can spend more time with their patients.
The report, produced by ISD for the Scottish Government, states that the estimated headcount of GPs working in general practice has changed "very little" over time.
However, the estimated WTE of GPs has dropped from 3,735 in 2013 to 3,575 in 2017.
There has also been "a continued decrease" in the proportion of GPs working a full-time commitment of eight or more sessions per week, from 51% in 2013 to 37% last year.
"This decrease is occurring against a backdrop of increasing demand on GP services, with a continuing drive to shift from hospital to primary care and an ageing population," the report said.
Nearly one in four (24%) of responding GP practices reported current GP vacancies, compared to 22% in 2015 and 9% in 2013, the publication stated.
Almost nine out of 10 practices reported using GP locums in the 12 months before the survey was carried out.
The WTE of registered nurses and healthcare support workers employed by general practice increased, however, between 2013 and 2017, by 9% and 33% respectively.
RCGP Scotland said it has been warning for several years that the GP workforce is in "crisis".
"The findings from this survey are very concerning," chair Dr Carey Lunan said.
She added: "We need action to tackle the GP shortage sooner rather than later to ensure that GPs can continue to deliver high-quality, safe patient care.
"Many practices across the country are already at tipping point. Growing the GP workforce is crucial to ensure that there are enough highly trained medical generalists to provide the community-based, patient-centred care that has provided the backbone of the NHS for decades."
Dr Alan McDevitt, chair of the British Medical Association's (BMA's) Scottish GP committee, said the vacancy figures show that recruitment and retention problems continue to be an issue.
"Every unfilled vacancy puts more and more strain on remaining GPs who must struggle to cover the gaps in their practice while also coping with rapidly increasing demands on GP services," he said.
He added that the new GP contract, to be implemented in April, will start to address the workload pressures being felt by GPs and their staff.
Health Secretary Shona Robison said: "Primary care workforce levels in Scotland are now at a record high and it is especially encouraging to see the numbers of practice nurses have increased by almost 10% and healthcare support workers by a third in the last five years.
"This is in line with our vision of expanding teams of healthcare professionals working in general practice such as practice nurses, physiotherapists and pharmacists, so patients can access the right person at the right time.
"The new GP contract, backed by investment of £110 million, will ensure GPs can spend more time with the patients who need them most and make general practice an even more attractive career, and our commitment to increase the number of GPs by 800 over the next decade will ensure a sustainable service that meets increasing demand."
A total of 774 general practices, 82% of those eligible, responded to the survey on "in hours" services.