The number of people working 'extreme' unpaid overtime of more than ten hours a week increased by 14,000 to nearly 900,000 last year, according a new analysis of official statistics published by the TUC to mark Work Your Proper Hours Day (WYPHD). Teachers and lawyers are the most likely to do 'extreme' unpaid overtime with around one in five employees clocking up an extra 17 hours of free work a week. Last year over five million people across the UK clocked up an average of 7 hours 12 minutes of unpaid overtime a week worth £27.4 billion - or £5,402 each.

The TUC analysis shows that teachers, health and social service managers and lawyers are most likely to do unpaid overtime, with around half of all staff in these occupations regularly giving away unpaid hours. One in four (25.3 per cent) public sector workers worked unpaid overtime in 2009, worth nearly £9 billion a year, compared to one in six staff in the private sector (18.3 per cent). Public sector workers are also more likely to do 'extreme' unpaid overtime, according to the TUC analysis.

Single women are more likely to do unpaid overtime than single men, married or cohabiting couples or lone parents - with over one in four women putting in an average of 7 hours 42 minutes free work a week.

The growth of 'extreme' unpaid overtime over the last year comes as the number of 'underemployed' people also increases. Official figures show that 2.8 million people say they want more hours in their existing job or full-time work instead of their current part-time job.

With official statistics showing that nearly half a million managers say they would be willing to work fewer hours even if it meant a pay cut, there is an obvious mismatch between the kind of hours people want to work and the kind of hours they are getting, says the TUC.

Employees across the UK are putting in unpaid hours at work to help their company recover from the recession so the TUC is calling on bosses to mark Work Your Proper Hours Day by thanking staff for the extra work they are doing to help keep businesses afloat.

TUC General Secretary Brendan Barber said: 'One of the features of the recession has been people moving to shorter hours or taking part-time work in order to avoid dole queue. This has also led to a fall in the number of people putting in extra hours at work.

'There has also been a surprise increase in people doing 'extreme' unpaid overtime, with nearly 900,000 workers giving away 18 hours of free work a week last year. There is no direct link between excess overtime and underemployment but those people who are struggling to find enough or indeed any hours to work must be wondering why some workers are doing so much for free.

'Our analysis also disproves the tired stereotype that public sector workers enjoy a feather-bedded working life. In fact they are giving away billions of pounds worth of work for free - and more per person than those in the private sector.

'Staff are understandably doing all they can to help their company recover from the recession - and bosses should thank them for going that extra mile. But working time still needs to be properly managed. A long hours culture is bad for workers' health and family life - whether the hours are paid or not.'