Freelancers account for more than two million of the 4.9 million people in self-employment in Britain, a group that now makes up 15% of all employment.
Official data on their earnings are infrequently published, something labour economists say is increasingly a blind spot in economic data because of the rapid growth of the self-employed sector.
The quarterly average earnings among the top categories of freelancers rose to 20,480 pounds in the April-June period from 20,474 pounds in the first quarter, a 0.1% increase, according to the Association of Independent Professionals and the Self-Employed (IPSE) trade body and online freelancer portal People Per Hour.
Official data on Tuesday showed Britain's workers received their biggest pay rises in more than 11 years this summer as the unemployment rate fell back to its lowest since the mid-1970s, even as the political crisis over Brexit deepened.
IPSE said confidence was draining from freelancers as the Brexit crisis escalates, chiming with surveys of businesses.
"We find a freelance sector that is flatlining in the face of Brexit," Ryan Barnett, IPSE's economic policy advisor said.
"Pay, too, has dropped sharply from the Q4 2018 surge, as has the amount of work freelancers are getting."
Average day rates charged by freelancers also barely rose in the second quarter, the survey showed.
The survey polled 955 managerial, professional and technical freelancers between June 26 and July 12.
(Reporting by Andy Bruce)