The PM also wants to widen the availability of flexible working for all jobs unless there are “solid business reasons not to” and increase female representation in at senior levels.
May said: “Tackling injustices like the gender pay gap is part of building a country that works for everyone.
“Already many of the UK’s top companies are leading the way in making sure everyone’s contributions to the workplace are valued equally, and it is encouraging news that the gap has fallen this year for full-time workers.
“But the gender pay gap isn’t going to close on its own – we all need to be taking sustained action to make sure we address this.”
“This announcement is a damp squib that will have little impact. Companies should be forced to publish their pay gaps – not merely encouraged.”
Based on median hourly earnings for full-time employees, the gender pay gap has fallen to the lowest level since 1997, dropping from 9.4% in 2016 to 9.1%.
Large companies including Weetabix, Fujitsu, TSB, Virgin Media and SSE have already reported their gender pay gap and have said how they will take action to close it.
May is now calling on more employers to follow their lead, including encouraging businesses with fewer than 250 employees to voluntarily publish their pay gap too.
She added: “We need to see a real step-change in the number of companies publishing their gender pay data and offering progression and flexibility for all employees.
“That’s why today I am calling on more businesses, both small and large, to take action to make sure the gender pay gap is eliminated once and for all.”
Despite positive action on the gender pay gap, the wider picture for UK workers is worsening.
Full-time workers’ weekly earnings have fallen in real terms for the first time since 2014, new figures revealed this week.
Despite wages increasing by 2.2% since last year, according to the Office for National Statistics, any potential gains have been eroded by inflation rising even further.
Median weekly earnings are now £550 a week - down 0.4% on 2016 when adjusted for inflation, the Press Association reported.