THERESA May is calling on more companies across Britain to publish details of their gender pay gap in a renewed drive to improve equality in the workplace.

The Whitehall drive comes as new figures this week showed that while the gender pay gap is closing across the UK as a whole, in Scotland it is rising.

The TUC dismissed the new initiative, branding it a "damp squib," and insisting employers should be forced to publish the numbers not merely asked to.

The Prime Minister insisted "sustained action" was required from employers if the differential between the pay of men and women was to be eliminated "once and for all".

The UK Government has already introduced a legal requirement for all employers with over 250 employees to publish their gender pay and bonus data by April 2018.

However, Mrs May is now appealing to smaller firms to disclose their details as well.

She is also urging companies to take steps to help women progress through the ranks of management, ensuring better female representation at senior levels.

The PM 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.

Her intervention comes after new figures from the Office of National Statistics showed that women employees in Scotland, whose average salaries are lower than those in England, are being paid on average 15.2 per cent less than men, a gap which is slightly wider than it was last year.

"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,” said Mrs May.

"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.”

The PM said a “real step-change” was needed 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," she added.

But Frances O’Grady, the TUC General Secretary, said the gender pay gap would continue closing at a snail’s pace unless the Government came down much harder on employers.

“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, which 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.”

The TUC noted how since 2011 the full-time pay gap had fallen by an average of just 0.2 percentage points a year, saying at this rate it would take around 40 years to achieve pay parity between men and women.

Between 1997 and 2010, it added, the full-time pay gap fell by 0.6 percentage points a year.

Commenting on plans to extend flexible working rights, Ms O’Grady said: “It’s high time more people are given access to flexible working. Millions of mums and dads are struggling to juggle childcare with work.

“But it’s a shame that workers on agency and zero-hours contracts won’t benefit from this policy. Ministers must help them too,” she added.