More than five million people worked an average of 7.6 hours a week, effectively having £6,828 taken out of individual pay packets.
Excessive hours are a risk factor for the development of heart disease, stress, mental illness, strokes and diabetes.
The analysis was published on 28 February, the TUC’s 16th annual Work Your Proper Hours Day, which marks the date on which the average person who does unpaid overtime has effectively worked the year so far for free.
One in four public sector employees worked unpaid overtime, compared with around one in six employees in the private sector (16.4%).
More than one in six (18.3%) men work unpaid overtime, averaging 8.2 hours per week. A similar percentage of women (18.8%) also put in unpaid hours. Even though many women work part-time the average for those undertaking unpaid overtime is seven hours a week.
London relies most on free work, the TUC found, with 24% doing unpaid overtime, which compares with the national average of 19%.
The South East is next, with 22% working unpaid overtime, while 19% in the South West and 18% in the Eastern Region and West Midlands are working free hours.