All large companies would be required by law to reveal the gender pay gap between their male and female employees, under plans announced by Liberal Democrats designed to put pressure on them to move towards equality.
The scheme - which creates a new dividing line with Conservative coalition partners, a day after Lib Dems announced plans to scale back the so-called "bedroom tax" - will be included in the party's manifesto for next year's general election, leader Nick Clegg said.
Only 140 firms have so far signed up to a voluntary scheme introduced under the coalition, and Mr Clegg said it was time to accept that this approach "does not go far or fast enough".
Lib Dem sources said that the party had been trying to persuade Tories to move to a compulsory scheme for the last two years but no longer believed there was any prospect of securing agreement across the coalition before the general election.
The bid to woo women voters marks a further attempt by Lib Dems to restore their poll ratings and stave off the prospect of electoral meltdown in 2015, after consistently polling in single figures over the past few months.
The Lib Dem plan would put into action measures included by Labour in the 2010 Equalities Act but never implemented.
Under the scheme, all companies employing over 250 people would be required by law to publish the average pay of male and female employees.
Lib Dems will consult on the precise details and timings of publications, but companies which fail to comply can expect to be fined up to £5,000.
Mr Clegg said it was "utterly unacceptable" that the median average hourly pay for women in 2013 was just £10.33, compared to £12.97 for men.
"Forty years after the Equal Pay Act was passed it is utterly unacceptable that women are not being equally rewarded in the workplace - with women paid, on average, 20% less than men," the Deputy Prime Minister said.
"Real equality means fair pay.
"It's time to accept that the voluntary approach does not go far or fast enough.
"We need to lift the lid on what big companies pay the men and women they employ, with that information there for every employee and customer to see."
Lib Dem sources said the gender pay rules would be applied only to larger employers in the private and voluntary sectors, in order to avoid imposing additional regulatory burdens on small and medium-sized enterprises.
Government departments already published similar information.
The 7,500 companies affected represent 0.13% of Britain's total, but together employ millions of people.
Party sources said that the cost for each company would be relatively small - around £100 a year - as the necessary information would already be held by accounts departments.
Equalities minister Jo Swinson said: "The Liberal Democrats have fought for shared parental leave, extra childcare, a new right to request flexible working, and we are determined to tackle the issue of gender pay.
"If women in the workplace are to have the same opportunities and choices as men, they must be properly rewarded for their talents and skills - it's as simple as that."
Business Secretary Vince Cable said: "We're consulting with business to make sure we get the detail right, but ultimately this is a good step for our companies.
"We've already seen some of our biggest firms lead the way on publishing pay gap information.
"They know that their staff will appreciate real openness about the way men and women are paid and real effort to close any gap that exists."
But Labour equalities spokeswoman Gloria de Piero said: "The reality is that under this Government the gender pay gap is back on the rise and women across the country are poorer as a result.
"The Lib Dems have had four years in Government to deliver pay transparency, they even have a Lib Dem minister in charge of the department responsible.
"But instead they've refused to implement the power Labour put in place to do this and their 'groundbreaking voluntary approach' has been a total flop.
"Women will know these words are meaningless as you can't trust a word the Lib Dems say."