There are three "main problems" contributing to the pay inequality between men and women, according to the project manager at the Fawcett Society.
Jenny Westaway from the campaign group told Channel 4's News at Noon the first reason for the gender pay gap is that women enter careers that are generally "valued less" than the jobs performed by men.
The second reason she outlined was the fact that women have more responsibility at home, "which means they are not able to compete with men on the same basis in the workplace".
This problem is compounded by a culture of longer working hours in the majority of places of employment, Ms Westaway added.
"Number three it is just straight discrimination against women," she concluded.
The Fawcett Society, which has been campaigning for women's rights since 1866, has recommended that businesses should undertake salary audits to ensure male and female employees are being paid the same.
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