Companies are being urged to stop relying on the generosity of their staff after a study revealed workers are putting in billions of pounds worth of unpaid overtime.

Research by the TUC revealed 5.3m people worked an average of 7.7 hours a week in unpaid overtime last year, worth £33.6bn.

The union organisation has asked businesses not to become reliant on staff working for free, and to make sure employees take proper lunch breaks and leave on time.

Chief executives top the list of those doing the most unpaid overtime, averaging 13.2 hours a week, followed by teaching staff (12.1 hours), finance managers (11.3 hours) and managers in production and health care (10 hours).

The TUC dubbed today Work Your Proper Hours Day, saying the average employee doing unpaid overtime had effectively worked for free so far this year.

The TUC warned that working time protections could be weakened when the UK left the EU.

General secretary Frances O'Grady said: 'Few of us mind putting in some extra time when it's needed, but if it happens all the time and gets taken for granted, that's a problem.

'So make a stand today, take your full lunch break and go home on time.

'The best bosses understand that a long-hours culture doesn't get good results. So we're asking managers to set an example by leaving on time too.

'The government still doesn't have a water-tight plan to stop working time protections getting weaker when we leave the EU.

'The prime minister should promise to put a guarantee into our future trade deals with Europe that British workers will have a level playing field with EU workers.'

Workers in London did the most overtime, followed by those in the South East and South West.

Kevin Courtney, general secretary of the National Union of Teachers, labelled the situation 'untenable'.

He said: 'Once again, the TUC has found that teachers and the education sector as a whole are subject to enormous levels of unpaid overtime.

'Long and unmanageable working hours are the biggest single reason cited by teachers for leaving the profession.'

A Department for Education spokesman said: 'We have taken action to address the key areas that teachers told us generate unnecessary work – marking, planning and data management – and we intend to take further steps to help address it through our action plan.'

Matthew Newnham, an employment partner with Birketts in Norwich, said there was a 'grey area' around salaried employees who worked extra hours.

'If you are an employee who earns a salary, you can be expected to work as many hours as are required to do the job, which may be more than your core hours.

'Someone on the factory line who clocks in and out, if they are asked to work an extra hour, that is an overtime situation. There are not many people who are paid by the hour who would work for free.'

He added: 'It is acknowledged that those who work hardest and longest are likely to get the pay rises and the bonuses.'

Working hours – the legal guidelines

UK employers are legally obliged to follow the Working Time Regulations – the British implementation of the European Working Time Directive.

These rules state employees are not allowed to work more than 48 hours a week in any 17 week period, but they are able to sign a waiver of this right.

Workers must have an 11 hour break between shifts, and must have at least 24 hours off in any seven day period.

They are also entitled to a 20 minute break on a shift of six hours or more.

If employers fall foul of these laws, the health and safety executive will step to investigate their practises. Individual complaints from employees about overtime and hours can be taken to an employment tribunal.