Almost half of women who have worked while pregnant had experienced some form of unfair treatment, research for the Equal Opportunities Commission (EOC) has found. New and expectant mothers said that they had been denied promotion and bonuses, had faced verbal abuse and had been stopped from attending training courses.
The EOC called for urgent action to end the unlawful treatment of pregnant women at work and said its research made for "shocking" reading.
About 441,000 women work while pregnant every year, but the EOC survey of 1,000 working mothers found disturbing evidence of maltreatment. Nearly half (45 per cent) said they had experienced some form of discrimination while pregnant or on maternity leave.
One in five said they had lost out financially and one in 20 disclosed they were put under pressure to hand in their notice when they announced they were expecting a baby. Seven per cent said that they were either sacked, made redundant or left their jobs because they faced discrimination.
The EOC chairwoman, Julie Mellor, said: "Women should not be penalised simply for being pregnant. The impact on women, their partners and families, and on the health of their baby can be disastrous.
"We need urgent action from the Government to provide more information and support for pregnant employees and their employers."
The EOC called on the Government to provide a written statement of maternity rights and employer responsibilities to every pregnant woman.
About 1,000 unfair dismissal cases related to pregnancy are registered at employment tribunals in England and Wales each year.
Union leaders said that many women's cases never came to light because the stress of having a new baby and leaving work meant that mothers were often reluctant to take their bosses to a tribunal.
Debbie Coulter, deputy general secretary of the GMB general union, said: "The reality is that the British economy needs women working as never before. The majority of women who are affected are unlikely to take further action against their employers because of stress, cost and time. The last thing you want to do while pregnant or you are caring for a new baby is go to an employment tribunal.''
More needed to be done to change employers' and colleagues' attitudes to pregnancy in the workplace: "It is too often viewed as an expensive inconvenience, as is maternity leave," she said.
Several high-profile cases have highlighted discrimination suffered by working women when they become pregnant. A pilot at British Airways appeared at a tribunal last month, claiming the company discriminated against her by refusing to let her work part-time after the birth of her daughter.
That case is continuing, as well as another in which a female banker claims her boss told her that pregnant women should "stay at home and take piano lessons".
But the Federation of Small Businesses (FSB) called for mothers and the Government to do more to solve the problems surrounding maternity pay.
Peter Firth, a member of the FSB who sits on the EOC's pregnancy taskforce, said: "Small businesses genuinely employ the best person for the job, irrespective of their age and gender.
"The reality is that mothers and pregnant women can experience discrimination in the workplace, but the reality is also that employers find administering maternity rights a headache."
He said the system needed an urgent overhaul: "The Government should take back responsibility for paying statutory maternity pay and open the door for more dialogue between mother and employer about when she expects to return to work."