Campaigners highlight 10 November as the point in 2017 when a woman on an average wage stops being paid relative to their male counterparts.
But in some parts of the UK, the gender pay gap is so wide, it is as though women work unpaid from September.
Vivienne Hayes, of the Women’s Resource Centre, said progress had moved at a “snail’s pace”.
Campaign group the Fawcett Society said that progress in closing the pay gap has “stalled”.
If the mean average pay gap for full time workers of 14.1% closes at the rate it has over the last five years, it won’t reach 0% until 2117, it said.
The government wants large firms to disclose their pay gap, but will not force them to comply.
Ms Hayes, chief executive of charity the Women’s Resource Centre, said: “We are here again, year after year lamenting the seemingly impervious issue of equal pay for men and women.
“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.”