Women are in theory working for free from today until the end of the year because of the gender pay gap, campaigners say.
November 10 marks the point in which women on an average wage stop being paid relative to their male counterparts, research suggests.
The date, which has remained the same for the last three years, reflects the mean pay gap for full-time workers between the genders.
The mean stood at 14.1 per cent in April this year, according to the Office for National Statistics.
Progress made to close the gap has moved at a “snail’s pace”, Vivienne Hayes, of the Women’s Resource Centre, said.
If the mean percentage that represents the imbalance continues to decrease at the rate is has over the last five years, it will not reach 0 per cent until 2117, according to campaign group the Fawcett Society.
It said the gap was wider than the average for women working full time in their 50s, at 18.6 per cent, but that it has also grown among women in their 20s - from 1.1 per cent in 2011 to 5.5 per cent this year.
"We are here again, year after year lamenting the seemingly impervious issue of equal pay for men and women,” Ms Hayes, chief executive of charity the Women's Resource Centre, told the BBC.
"Even though we have had a law since 1970 outlawing the practice of sex discrimination in pay, our progress is probably not even at a snail's pace."
According to the Financial Times, London has shown the highest rate of unequal pay compared to all other UK regions - at 20.7 per cent.
While the gap has decreased by more than three per cent in the capital and the South-East since 2011, it has risen by 1.5 in the North East.