The Industrial Society says when companies discriminate against women they do so for so-called "rational" reasons, because men cost less to employ.
It argues that the government's introduction of increased maternity leave without bringing in similar rules for new fathers has pushed many firms into taking on men instead.
The society concludes that the government's attempts to improve the lot of working mothers are backfiring, as many employers are now reluctant to take mothers on.
There has been a big rise in the number of working mothers in recent years. Only one in five new mothers worked 20 years ago, but today around half of all mothers with a baby aged up to one-year-old work.
Helen Wilkinson of the Industrial Society said: "What we are saying is that the government is not being radical enough.
"Whilst we have a scheme of paid maternity leave and unpaid parental leave we are discriminating against men as fathers in the workplace."
She called for the government to give new fathers as much time off as new mothers.
'Parallel rights'
"We are also reinforcing gender inequality between women and men and reinforcing the pay gap, hence women are not actually getting the equality that they deserve in the workplace.
"Delivering equality for women in the workplace actually means giving men parallel rights as parents."
She added that the UK should follow the example of Scandinavia, where new fathers are given a non-transferable period of leave which they must take or forfeit.
Mary Ann Stephenson of gender equality group the Fawcett Society said: "Women will not be able to achieve equality in the workplace until they have equality in the home.
"Employers often complain, they complained when women had equal pay they would no longer employ them, they complained when maternity leave was first introduced they would no longer employ women.
"Actually employment amongst women has gone up."