A record 5.24 million people worked unpaid overtime last year according to the Trade Union Congress (TUC), who says fear of redundancy could be driving employees to put in the extra hours.
It estimates the combined value of the extra hours is worth £26.9bn, and says the figure is the highest number since record began in 1992. The previous record was five million in 2001.
The TUC calculations are based on analysis of official statistics and show employees would receive an extra £5,139 a year if they were paid for the additional hours they are putting in.
It also reveals that if all the unpaid work was done at the start of the year, the first day employees would get paid would be 27 February.
Brendan Barber, TUC general secretary, said: "While some of this is due to the long-hours culture that still dogs too many British workplaces, the recession will now be making many people scared of losing their job in the year ahead and joining the ever-growing dole queue. Inevitably people will be putting in extra hours if they think it can help protect against redundancy or keep their employer in business."
The biggest increases in unpaid overtime have taken place in London ( 79,000), the East Midlands ( 61,000) and Eastern England ( 53,000). The number of people working unpaid overtime fell in the South East (-26,000) and Scotland (-11,000).