A total of 31% of around 600 respondents to a poll by GPonline's sister website Medeconomics said fees income had dropped over the last year.
Just 9% said their income from private and professional fees had risen, while 37% said it had stayed the same and 23% did not know.
Around a quarter of practices said that over the coming year they planned to increase income from fees, which practices can charge for completing work including insurance reports, cremation forms and licensing reports not covered by NHS funding.
GP private fees
Some practices reported that they planned to review or increase charges, while others said they would carry out more work that attracts fees, or become more consistent on their charging policy.
Laurence Slavin, a partner at specialist medical accountants Ramsay Brown and Partners, told GPonline that almost every practice he spoke to said fees had fallen this year, by up to 20% in some cases.
'Income from fees is certainly down, by around 10% on average,' he said.
Private and professional fees income makes up a small proportion of overall practice funding, but a dip in this income adds to a wider trend that saw GP profits fall 3% in 2003/14 and nearly 25% in real terms since 2005.
Mr Slavin said insurance companies were often using their own people to complete medicals that GPs would have been paid for in the past, with GPs now just asked to copy notes for smaller fees.
'Total income from private fees is maybe £5,000 per partner,' he said. 'So the losses for practices are up to £1,000 per partner per annum.'
Many respondents to the poll admitted their practice had been relaxed in the past about charging for services that should attract private and professional fees, but that they now intended to tighten up.
Some said they would ensure that all partners charge, while others said they would adopt national fee rates. Some planned to offer these services via federations, or to offer the services via other practices as well as their own.