Businesses with more than 250 employees will have to report the gap between male and female pay under new rules expected to be introduced next year.
Now, David Cameron has said that will also include the amount paid as a bonus, and that the rules will be extended to apply to the public sector.
"You can’t have true opportunity without equality. There is no place for a pay gap in today’s society and we are delivering on our promises to address it," the Prime Minister said.
Women still earn on average 22 per cent less than men, the most recent figures reveal, amounting to women working an hour a day for free.
"One of the biggest drivers of gender pay discrepancy, especially at senior levels, is the bonus gap. Bonuses are also where gender bias can creep in easily as they are amongst the least transparent forms of pay," said Ann Francke, chief executive of the Chartered Management Institute.
"There’s a tendency to reward those in our own image or to think that because men may be the ‘main breadwinners’ they deserve higher bonuses. And men often negotiate harder or trumpet their achievements more readily."
Women's bonuses were found to be almost half the amount pocketed by men according to the research.
“Eradicating the gender pay gap is an important goal, but to be truly effective, gender pay gap reporting must be relevant to each company rather than a box-ticking exercise," said CBI employment and skills director Neil Carberry.
Occupational stereotypes also need to be challenged, he said, "by encouraging more women into male dominated industries and strengthening careers advice”.
“Companies recognise the value of having a diverse board that reflects society and their customers, so we look forward to working with the government to end all-male boards in the FTSE 350 and elsewhere, building on the successful voluntary approach pioneered by Lord Davies," added Carberry.