New league tables will also be launched giving details of companies failing to address the problem.
And ministers are taking action to make sure that thousands more girls study maths, engineering, science and technology at school.
Women and equalities minister Nicky Morgan said the government wanted to secure "real equality" for women and reduce the gap in pay between men and women.
"In recent years we've seen the best employers make ground-breaking strides in tackling gender inequality.
"But the job won't be complete until we see the talents of women and men recognised equally and fairly in every workplace.
"That's why I am announcing a raft of measures to support women in their careers from the classroom to the boardroom, leaving nowhere for gender inequality to hide.
"At the same time I'm calling on women across Britain to use their position as employees and consumers to demand more from businesses, ensuring their talents are given the recognition and reward they deserve."
A £500,000 package was announced aimed at helping the 8,000 employers who will have to publish their average pay and bonus gap between men and women.
The first league table will be published in 2018, making it possible for women to compare pay in different sectors.
The government said it aimed to have 15,000 more girls studying maths and sciences by 2020, a 20% increase on current numbers.
TUC general secretary Frances O'Grady said: "If David Cameron is serious about ending the gender pay gap within a generation we need a much bolder approach from ministers.
"While today's announcement is a step in the right direction, we're disappointed that firms won't have to publish their gender pay gap figures until 2018.
"It is a real shame that bosses won't be made to explain why pay gaps exist in their workplaces and what action they will take to narrow them."
The TUC said it was "shocking" that the gender pay gap was still over 19% for all workers and 9.4% for full-time employees, adding that at the current rate of progress it would take almost 50 years to close it.
Chris Benson from law firm Leigh Day said: "The government has finally woken up to the fact that the gender pay gap stubbornly persists, despite over 40 years of unequal pay being illegal, and recognised that steps need to be taken to minimise or remove that gap."
Carolyn Fairbairn, CBI director general, said: "League tables should not be used to name and shame firms, as data will only be able to present a partial picture, particularly given factors such as the mix of part-time and full working and sectoral differences.
"Where reporting can be useful is as a prompt for companies to ask the right questions about how they can eradicate the gender pay gap.
"The government should consult closely with business to ensure that this new legislation helps close the gender pay gap, rather than ending up as a box-ticking exercise."
Kate Green, shadow minister for women and equality, said: "2018 is far too long for women to wait when the UK gender pay gap remains well above the EU average.
"At this rate it will be another 47 years until the gap is closed, so we haven't a moment to lose. The Tory government's approach simply isn't good enough for women.
"While Nicky Morgan likes to claim there is nowhere left to hide for gender inequality, it is her government's policies that have consistently worsened and entrenched economic inequality for women."
Ann Francke, chief executive of the Chartered Management Institute, said: "Shining a light on what men and women are paid at every level, as well as monitoring the percentage of women at every level, is proven to speed up progress.
"Today, women make up 60% of junior managers, 40% of middle managers, and 20% of senior managers.
"Transparent reporting of pay at every level will tackle the 'glass pyramid' that stifles potential and productivity in business."
Rebecca Hilsenrath, chief executive at the Equality and Human Rights Commission, said: "Today's announcement is a welcome, but long overdue, step in the right direction.
"Mandatory reporting should encourage large companies to take action to close their gender pay gaps.
"However, while we welcome this requirement for more transparency, we are concerned that the proposals do not appear to provide any sanction for failure to comply.
"If companies are to take this seriously, we need to be able to use our enforcement powers to ensure that they do what is required."
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