Three out of four working mothers have been affected by pregnancy and maternity discrimination, a "worrying" new study has revealed.
Research among 3,000 mothers showed that many were not told about promotion opportunities, were denied training or even threatened with dismissal.
Only one in four women affected by discrimination raised it with their employer and fewer than 1% took a case to an employment tribunal, the survey by the Equalities and Human Rights Commission found.
The £1,200 cost of taking a case to a tribunal was blamed, as well as a lack of information and the stress of pursuing a claim.
The Government was pressed to take urgent action to tackle the problem, including preventing employers asking during job interviews about a women's intention to have children.
Caroline Waters, deputy chairwoman of the EHRC: "We simply cannot ignore the true scale of the hidden discrimination that working mothers face. This is unacceptable in modern Britain, and urgent action is needed to ensure women are able to challenge discrimination and unfairness. This is why we are calling on Government to look at the barriers working pregnant women and mothers face in accessing justice.
"We want to make workplaces fairer for everyone and get rid of outdated practices like asking women during job interviews whether they intend to have children. For businesses to thrive, they need to harness the talents, skills and experience of all employees.
"We are calling on employers, regulatory bodies and the voluntary sector to make vital changes needed to improve the lived experiences of British workplaces so they are the best they can be for everyone."
MP Maria Miller, who chairs the Women and Equalities Committee said: "This report provides hard evidence that there is widespread and worrying discrimination against women during pregnancy and when they return to work from maternity leave.
"This is despite the legal protections that have been in place for the past 30 years. My committee is very concerned by these findings and is launching an inquiry to follow up on the report and recommendations."
Around 3,000 employers were also questioned, with one in four believing it was reasonable to question women during a job interview about plans to have children.
Three in four mothers who did not get a job after being interviewed while pregnant believed it had affected their chance of being employed.
Around 4% of mothers said they left a job because of health and safety risks and one in 10 had problems getting time off for antenatal appointments.
Just over half of employers gave no training or support to managers on dealing with pregnancy and maternity issues.
The commission called on the Government to change the tribunal fee system so they were not seen as a "barrier" for pregnant women and mothers.
The report noted that, since the introduction of fees in 2013, the number of sex discrimination cases has dropped by 76% and pregnancy-related cases by 50%.
An insurance scheme should be set up to help smaller firms deal with maternity leave, it was suggested.
The commission also recommended increasing the time limit for a woman to bring an employment tribunal case involving pregnancy and maternity discrimination from three to six months.