The average female cocoa farmer is paid as little as 23p a day, highlighting a gender pay gap in the global chocolate industry, according to Fairtrade.
That figure is well below the extreme global poverty line of £1.40 a day, and average farmer pay rate of 75p a day.
The group is calling on the government and chocolate industry to join an alliance to increase women's pay.
It is using Fairtrade Fortnight to highlight the "hidden inequality" of females in the chocolate industry.
The foundation says the UK chocolate industry is worth at least £4bn a year, with Brits consuming more per person than any other European country.
And that, it says, means the UK should be leading efforts to ensure that all cocoa farmers and chocolate workers, especially women, can earn a living income.
The group has called on the UK government and chocolate industry to join the Alliance on Living Incomes in Cocoa, a new international initiative.
The BBC has contacted the government for a response.
Fairtrade has launched a She Deserves campaign, revealing that in West Africa, where 60% of the world's cocoa is grown, the average female cocoa farmer earns as little as 23p a day.
In the Ivory Coast, despite carrying out 68% of the labour, which involves planting and harvesting, hacking cocoa pods, fermenting, drying and bagging up the cocoa beans - as well as domestic duties in the home - women have fewer rights than men, receive less money and are often landless.
"Often the woman does two thirds of the work for less than a third of the income, meaning a bitter taste to the sweet treat," Louisa Cox, director of impact at the Fairtrade Foundation, said.
"If the cocoa industry is serious about a long-term sustainable future for their business then they must truly sweeten the deal and invest more in the women behind our chocolate."
Julia Nicoara, director of public engagement at the Fairtrade Foundation, said: "Many of us don't know the bitter truth of exploited farmers behind much of our chocolate, with women doing much more of the work for much less of the pay."
A series of events will be held across the UK in the next two weeks, with the Fairtrade Foundation's grassroots networks of around 1,600 Fairtrade schools and towns staging storytelling events to highlight the women's lives.