The Trades Union Congress is urging anyone who often does unpaid overtime to work their proper hours tomorrow. It also wants British bosses to take their staff out for lunch, coffee or a cocktail to say thank you for their workers' contribution.

UK employees worked unpaid overtime worth £23 billion in 2004, according to a TUC analysis of official statistics published today. On average, each employee who worked unpaid overtime would have earned £4,650 for their unpaid hours if paid at their normal hourly rate. If they had done all their unpaid overtime at the beginning of the year, they would have worked for free until Friday 25th February.

In order to mark the occasion, the TUC is therefore urging employees to work only their contracted hours on Friday, and is encouraging bosses to express their thanks with lunch, coffee or a cocktail.

"We're not saying that we should turn into a nation of clock-watchers. Most people do not mind putting in some extra time when there's a crisis or an unexpected rush. But too many workplaces have come to depend on very long hours. They get taken for granted and staff have to do even more if there is an unexpected rush," said TUC General Secretary Brendan Barber.

The TUC, Time Out magazine and restaurant booking service toptable.co.uk are providing suggestions on how employees can fill their spare time on Friday.

Employees can also visit the TUC web site workyourproperhoursday.com to submit their manager's e-mail address. Tomorrow morning their boss will be sent a 'Bossagram' – a message suggesting that he should show some gratitude for their unpaid overtime.

The identity of the person submitting their manager's e-mail address will remain anonymous. Only one e-mail will be sent to any e-mail address, however many times it is submitted, and the text of each e-mail will be identical.

"The bossagram is a fun way to make a serious point. Too many workplaces are in the grip of a long hours culture, and with a bit better organisation people could gain extra hours of free time and still get their work done well," said Barber.

Emma Grossmith, an employment law specialist with Pinsent Masons, commented:

"Give and take is part of every healthy employment relationship and most people accept that from time to time they may have to work late. It is where this practice of unpaid overtime becomes the norm, rather than the exception that it can cause problems.

"If employees have no real control over whether they do the work or not (perhaps because their volume of work is unmanageable or because there is a long-hours culture), their extra time may count as 'working time' under the Working Time Regulations. This means that their employers should be monitoring their employees' time properly and making sure that any employees who are working more than the maximum weekly working time under the Regulations have signed a valid 'opt out'. Employers who fail in this duty could find themselves facing an investigation by the HSE, instead of just a 'bossagram' from the TUC."

According to the TUC, workers in Britain work the longest hours in Europe, and of these, Londoners put in the longest hours – around an extra 7 hours 54 minutes unpaid overtime in a week. If paid for this they would have earned an extra £7,000 a year.

Teachers and lecturers worked the longest unpaid hours of all – an average of 11 hours and 36 minutes a week, according to the TUC, although this does not take into account the average 13 weeks' holiday enjoyed by the profession.

Corporate managers came in second, reports the TUC, working an average unpaid overtime of 9 hours and 48 minutes per week, worth £19,000 a year.

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