A survey of 500 managers by law firm Slater & Gordon showed that more than 40 per cent admitted they are generally wary of hiring a woman of childbearing age, while a similar number would be wary of hiring a woman who has already had a child or hiring a mother for a senior role.
A quarter said they would rather hire a man to get around issues of maternity leave and childcare when a woman does return to work, with 44 per cent saying the financial costs to their business because of maternity leave are a major concern.
The study also saw a third of managers claiming that women are not as good at their jobs when they come back from maternity leave.
Employment Relations Minister Jo Swinson said: "Pregnancy discrimination is illegal, immoral and completely unacceptable. There is no excuse for such attitudes from these employers, who frankly are dinosaurs. British business simply can't afford to lose out on half of the available talent pool.
"In any event, the introduction of Shared Parental Leave from April will mean men as well as women taking leave to look after their new baby.
"This will help to stamp out outdated stereotypes about who should do what, and let parents get on with making their own decisions about how they manage work and family life."
TUC General Secretary Frances O'Grady said: "It is illegal to not give someone a job on the grounds that they may have children in the future.
"Employers that do this are not only breaking the law but being incredibly stupid as they are missing out on many of the country's brightest young workers."