For each employee, this accounted for an average of 7.2 unpaid hours each week, and the equivalent of £7,200 lost in wages each year.
The number of people doing unpaid work has slightly increased from 2022, when 3.5 million workers undertook unpaid overtime.
Paul Nowak, general secretary of the TUC, said: “Most workers don’t mind putting in extra hours from time to time, but they should be paid for it.
“Some employers fail to record the overtime staff do. And when they don’t record it, they don’t pay it.”
Read more: Men working more overtime than women
Employers who do not pay for overtime are at significant legal risk, according to Ed Heppel, partner and head of employment at Rollits.
He told HR magazine: “An employee is not legally obliged to work beyond their contracted hours and in circumstances where the contract does not cover overtime, the employer and employee should seek to agree the remuneration.
“Employers should be aware that failing to pay an employee for overtime can cause significant risk. For example, if the additional unpaid hours cause an employee’s remuneration to drop below the national minimum wage the employer can be subject to criminal proceedings and naming and shaming by the Department for Business and Trade.”
He said Acas guidance confirms employers do not need to keep records of employees’ working hours.
He added: “However, they must keep records to confirm that they are complying with legal obligations including that employees are not working more than an average of 48 hours per week.”
Read more: Work is getting more intense, say over half of employees
The TUC found that one in six (16.7%) public sector workers worked unpaid hours in 2023, compared with one in nine (11.9%) in the private sector.
Teachers did the most unpaid overtime in 2023, with 40% of all teachers doing unpaid work, an average of 4.4 hours each week.
Gordon McFarlane, president of the Public Services People Managers Association, told HR magazine that employers in the public sector are juggling competing demands.
He said: “As employers, we work hard to try and ensure that workloads are managed and manageable.
“However, with shrinking budgets and rising demands, it is inevitable that colleagues will work additional hours. The commitment that we see means that there is a great deal of discretionary effort.”
“In some areas, and for some roles, overtime is paid. But this has to be managed.
“We continue to look at ways in which we can become more productive, modernising and streamlining where there are opportunities.”
