Women who have children could be paid as much as 20% less than their male counterparts, according to the TUC.

The organisation claims the gender pay gap more than trebles once women reach the age of 30, with general secretary Brendan Barber describing this as "an unacceptable penalty simply for having children".

"We all expect our wages to increase as our careers progress. But women's wages start to stagnate as early as their 30s," he said.

"Despite girls outperforming boys at school and at university, too many employers are still failing to make use of women's skills."

The study also found that women are twice as likely as men to be poor.