Women who choose career over family could still earn almost £200,000 less during their working lives than male colleagues in the same job, according to government research.

The study was commissioned to find out what effect motherhood had on female earnings.

But it discovered that many women were being paid less than men simply because of their sex.

The Equal Pay Act of 1970 was introduced to prevent exactly this inequality.

The research is likely to put pressure on the government to ensure the legislation is enforced.

The study was commissioned by Women's Minister Baroness Jay and carried out by the London School of Economics.

It found that women who took career breaks to have children could lose out on hundreds of thousands of pounds during the course of their working lives.

But even women who chose to put their career before having a family were earning much less than men.

This backs up figures from the Equal Opportunities Commission, which says that women get paid only 80% of the average hourly male earnings.

It says the UK's record on equal pay is poor - it came 10th out of 15 European countries surveyed.

Previous research has found that women are less aggressive about demanding and negotiating pay rises.

Juliet Mitchell, a gender studies expert at Cambridge University said motherhood plays an important factor.

"Women are taken out of the workforce at the exact period of time when people earn the most," she said.

"They bear and look after children in the middle years from the last half of the 20s to the late 30s which is traditionally a high earnings period."

Last year, the EOC launched a campaign encouraging women to find out what male colleagues earned and to fight for equal pay.

It also wants companies to carry out payroll audits to ensure female employees are not being discriminated against.