Patricia Hewitt has warned employers over discrimination against pregnant women.

The trade secretary and minister for women was speaking after an Equal Opportunities Commission report claimed expectant mothers were still facing prejudicial practices at work.

An EOC survey marking national pregnancy week on Monday found that 21 per cent of respondents knew someone who had experienced problems at work because of their pregnancy, despite discrimination being illegal.

In contrast, 84 per cent disagreed with the statement that if a woman becomes pregnant it shows that she is less dedicated to her career compared with a woman who chooses not to have children.

The EOC will now mount a full investigation into the problem.

Hewitt said managers were both acting unlawfully and making a wrong decision in the interest of their own organisation when they make this assumption.

"Employers who discriminate against pregnant women are breaking the law and could be liable to pay compensation," she said.

"They are being foolish and harming their own businesses by excluding talented women from the workforce.

"Since 1997, the government has put in place a wide range of additional support for pregnant mothers who work, increasing both paid and unpaid maternity leave, introducing flexible working rights for new parents, and reducing the qualifying period for these rights.

"My department welcomes any new research in this area and looks forward to the results of the Equal Opportunities Commission's investigation."

EOC deputy chairwoman Jenny Watson said the issue was one of the biggest facing women workers.

"The EOC's helpline receives more calls from women facing problems at work because of their pregnancy than on any other subject," she reported.

"We hear of quite appalling cases of women who have been demoted, disciplined or even sacked simply for having a baby.

"And this discrimination during pregnancy continues to happen despite a widespread understanding of pregnant women's employment rights. We urgently need to find out why."