More companies should publish details of their gender pay gap in a renewed effort to improve equality in the workplace, Theresa May has said.
Big companies will soon be required to publish pay gap data, but the prime minister now wants smaller firms to play their part.
Reducing the gap "once and for all" requires "sustained action" from employers, she said in a statement.
But TUC chief Frances O'Grady said action, not words, was needed.
'Real step change'
The government has already introduced a legal requirement for all employers with more than 250 employees to publish their data on gender pay and bonuses by April 2018.
But Mrs May is now appealing to smaller firms to disclose their pay details.
She also urged companies to take steps to help women progress through the ranks of management, ensuring better female representation at senior levels.
Mrs May said firms should strive to make flexible working a reality for all staff by advertising all jobs as flexible "from day one" unless there were "solid business reasons" not to.
On Thursday, figures from the Office for National Statistics showed the overall gender pay gap rose marginally from 18.2% in 2016 to 18.4% in 2017.
The gap for full-time workers, however, fell to a record low of 9.1% - down from 9.4% the previous years.
Mrs May said: "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.
"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."
However, Ms O'Grady, the TUC's general secretary, said the pay gap would continue to close at a "snail's pace" unless the government "comes down much harder on employers".
She said: "This announcement is a damp squib that will have little impact. Companies should be forced to publish their pay gaps - not merely encouraged.
"Real action would be fining businesses who do not share information on what men and women are paid, and making firms explain what steps they are taking to close their pay gaps."