MPs and gender equality activists have put their names to an open letter calling for Prime Minister Theresa May to close the gender pay gap.

Woman effectively work for free from today (Wednesday, November 10) until the end of the calendar year because on average they earn 13.9 per cent less than men, gender equality charity the Fawcett Society claims.

Mitcham and Morden MP Siobhain McDonagh and Tooting MP Dr Rosena Allin-Khan were among 36 signatories, including journalists and political activists, of today’s letter to Theresa May.

Despite the Equal Pay Act being passed 45 years ago, women working full-time earn 13.9 per cent less than men in Britain today, according to the most recent figures from the Office for National Statistics’ Annual Survey of Hours and Earnings.

The Fawcett Society argues that “discrimination, undervaluing roles predominantly done by women, dominance of men in best paid positions and unequal caring responsibilities” are among the most important factors contributing to the pay gap.

Sam Smethers, chief executive of the Fawcett Society, said: “A root cause of the gender pay gap is that we don’t value the work done by women.

“As we mark Equal Pay Day this year we are focusing on the fundamental question of who and what we value and asking why it is that we don’t value women and the work they do – paid or unpaid.

“Equal value goes to the heart of the fight for pay equality, because the reality is that if it is a sector dominated by women the pay will be lower.”

Women make up over 60 per cent of those earning less than the living wage set by the Living Wage Foundation, according to a 2013 study by the Institute for Public Policy Research.

The gender pay gap for graduates ten years after earning their degree is 23 per cent, according to data compiled by the Institute for Fiscal Studies last year.

The Fawcett Society claim that men continue to dominate the most senior and best paid roles, citing the fact that last year, only five chief executives of FTSE 100 companies were women.

In today’s open letter to Mrs May, the 36 activists cite women being stereotyped as caregivers, girls being “steered towards less well-paid roles”, and a concentration of men at the top and women at the bottom of workplace hierarchies.

It reads: “Today, we are calling on the UK Government to close the gender pay gap.

“It would be good for the economy. But more than that, women are undervalued and that has to change.

“It’s time to end pay inequality once and for all.”

The Fawcett Society encourages people to tweet about Equal Pay Day using the hashtag #EqualValue