New research has found that on average the UK's freelancers and micro-businesses only get paid for two-thirds of the work they do for customers.
Online accounting software specialist FreeAgent analysed data on more than 40,000 businesses and found that a third did work for free in 2015.
The data shows that of the 10 million hours recorded and tracked by FreeAgent's customers in 2015, 80% were classed as "billable" and yet 31% of this time was not billed.
It found that on average small firms and sole traders only get paid for 69% of the work they do for clients.
In addition, the research found that the UK has seen a 15% increase in people working more than 48 hours a week.
Ed Molyneux, ceo and co-founder of FreeAgent, said: "For many UK freelancers and micro-business owners, the typical nine-to-five just doesn't exist. They often work irregular and long hours, which they may not always get paid for, and on top of this they also have the added pressure of staying on top of their business finances and tax obligations."
Molyneux said it is "surprising" to see how much work is effectively being done for free. "Many business owners invest some of their own personal time into the projects they work on, which they don't bill their clients for," Molyneaux said.
"But we believe there are also many who don't feel comfortable charging clients for important admin such as meetings and research, which they actually should be including in their bills. By doing so, they may not be working as profitably - or as efficiently - as they could be."