While the amount of cash GPs are allocated to provide services has gone up in recent years, the increases have failed to keep pace with the NHS budget as a whole. In 2012-13, 7.5 per cent of total spending was allocated to primary care, compared to a high of 9.1 per cent in 2005-06.
The Scottish Government insisted funding for GPs had gone up in real terms by 17 per cent over the ten years. However, doctors' representatives warned they were struggling to cope with soaring demand and costs, as the profession faces a recruitment crisis.
The Royal College of General Practitioners (RCGP) says family doctors services are nearing "breaking point" and has launched a campaign for funding to be raised to 11 per cent of the total budget within three years.
Scottish Liberal Democrat health spokesman Jim Hume said: "GPs are facing unsustainable pressure, with larger patient lists and growing demand for their services. Since the SNP came into office they have allowed the proportion of NHS spend on GPs to dwindle from over nine per cent to 7.5 per cent.
"When put into real terms this is no small sum of money. As we move to integrate health and social care, GPs will become even more important to their patients. The RCGP campaign reminds us of the real value GPs have to our local communities.
"SNP ministers need to respond more clearly and deliberately to their calls for more support."
Primary care spending soared from £519 million to £628m between 2003-04 and 2004-05, after GPs negotiated a new contract that meant they could opt out of providing out-of-hours care. Although they received slightly less basic pay, it meant they could top up their salaries by hitting targets or carrying out optional shifts.
Last year, the average wage for a GP in Scotland, the vast majority of whom work as independent contractors rather than directly for the NHS, was £88,700 per year.
In 2012-13, primary care services in Scotland were allocated £756m of NHS cash. In March, Health Secretary Alex Neil announced £6m of additional funding for primary care, but GPs at the Scottish Local Medical Committee annual conference said the boost would not keep pace with the soaring costs of staffing and running a practice.
The Government said it could not yet provide details of the proportion of its budget allocated to primary care in 2013-14.
Dr Alan McDevitt, chairman of the BMA's Scottish GP Committee, said workload would continue to increase as the population ages. He added: "There is an increasing drive towards care closer to home and a shift in the balance of care towards general practice but it is difficult to see how this can be sustained given that the proportion of spend from the NHS budget on general practice has fallen.
"If our politicians are serious about their commitment to NHS general practice, if they want to succeed in their agenda to provide more care to patients closer to home and if they really want integration to work, they need to invest in Scottish general practice."
The RCGP has said the number of annual GP consultations in Scotland had risen from 23.2 million in 2008 to 27.6 million in 2013, with the figure expected to hit more than 31 million by 2017. Family doctors are regularly working 11 hours a day and seeing 60 patients in a single shift, the organisation said.
Dr Miles Mack, a Ross-shire doctor and RCGP Scotland chairman elect, said: "20 years ago most of my work was with people coming in with short-term illnesses. We have taken on a large amount of chronic disease management and anticipatory care. We are up to the job, we are just looking for the resources to deliver it."
A Scottish Government spokeswoman said health boards had been asked to increase funding for primary care services, with general practice "at the heart" of its vision for healthcare.
She added: "This government has also increased the number of GPs by 5.7 per cent and has invested more than £757m to deliver primary care services last year, an increase of more than 17 per cent since 2004, with a further increase by £8m in funding to general practice this year."