According to research from the Chartered Management Institute (CMI), mothers are being discriminated against in the workplace and cornered into "softer roles". The CMI have dubbed this the "motherhood penalty".
The campaigners, who were speaking during the Gender Pay Gap Inquiry on Tuesday 15 December, also said the pay gap widens following on from women's childbearing years.
They found the pay gap between genders in full-time management and professional roles increased from 6% for 26 to 35-year-olds, to 20% for 36 to 45-year-olds.
Researchers from the CMI also told MPs that employers saw returning mothers as being of lesser value, and offered them part-time work instead of flexible hours.
CMI’s chief executive Ann Francke said, according to the Mail: "We found that mothers returning to the workplace were often shepherded into part-time or less challenging roles to fit in with childcare from well-meaning but misguided managers.
"They are not doing anyone a favour by offering softer roles for mothers.
"Capability must not be judged on time served; it is simply a question of whether you’re up to the job. If you are, you must be paid the going rate.
"Anything else is simply discrimination."
Rosalind Bragg, director of Maternity Action, a charity committed to ending inequality for mothers in the workplace said an urgent plan of action is needed to tackle this issue.
She told HuffPost UK Parents: "It is unsurprising that the gender pay gap widens following on from women's childbearing years.
"While there are laws in place to protect women in the workplace during their childbearing years, they are not being followed.
"Recent government research found that 54,000 pregnant women and new mothers each year lose their job because of unfair and unlawful treatment. This is 11% of all pregnant women in the workforce.
"Finding a new job when you are pregnant or caring for a young child is challenging. Women often face overt discrimination from employers and many are unable to find jobs at the same level of pay and responsibility."
Bragg said there is an "urgent need" for the Government to tackle the high rates of discrimination against mothers in the workplace.
She added: "We should focus on preventing unfair and unlawful treatment of pregnant women and new mothers at work, rather than relying on individual women to pursue expensive legal action against their employer."