Gloria De Piero, shadow minister for women and equalities, claims the Conservatives are "turning the clock back" for female workers by failing to do more to reduce the disparity.
In the latest in a series of summer interventions by the Shadow Cabinet, the former journalist and TV presenter will publish figures showing women still earn just 80p for every pound men take home.
The statistics - from the House of Commons library - will also reveal that the pay gap has closed at a rate of only 0.3% per year since 2010. That means with the current difference of 19.7%, it could take more than 60 years to deliver equal pay for women, she says.
By then, more than a century will have passed since the Equal Pay Act was introduced in 1970.
Ms De Piero, who represents Ashford, said: "Women should not have to wait more than 100 years for equal pay.
"The Tories are turning the clock back for women. More women are struggling to make work pay, earning less than the living wage and facing sky-high childcare costs. This isn't the progress our mothers, aunties and sisters fought for.
"When David Cameron claims working women have never had it so good, women know this isn't a government that speaks for them."