British companies must dramatically ramp up efforts to close the gender pay gap, equality campaigners said on Thursday ahead of a midnight deadline for organisations to report pay differences between male and female staff.

This is the second year that all companies and charities in Britain with more than 250 employees - covering almost half the country's workforce - have had to report their gender pay gap.

Hundreds of organisations had not filed figures by Thursday afternoon. Those that fail to comply risk legal action.

The government will release details on Friday, but data already posted shows many of the country's big banks have pay gaps exceeding 30 percent, based on their mean hourly pay rate.

"It is disappointing, but not surprising, that there are so many employers ... with large pay gaps and that these pay gaps aren't being closed," said Sam Smethers, chief executive of the Fawcett Society, a charity that campaigns on equal pay.

"The regulations are not tough enough. It's time for action plans not excuses."

Smethers said the government should make it compulsory for employers to publish strategies on how they will close their pay gaps.

She urged them to introduce more generous leave for fathers and make every job flexible, unless there was a strong business case not to.

The government encourages companies to publish plans, but they are not required to do so and as of May 2018, just under half had.

As in many other countries, gender pay inequality has been a persistent problem in Britain despite sex discrimination being outlawed in the 1970s.

Men in Britain earn on average 17.9 percent more than women, according to government data for 2018 – down from 18.4 percent in 2017.

The gap is partly driven by the fact more women work in part-time jobs, which are lower paid. The divide among full-time employees was 8.6 percent in 2018.

Business in the Community (BITC), a charity that promotes responsible business, also called on employers to publish their plans and share them with staff.

"The figures published so far this year clearly show that gender pay gap reporting on its own isn't enough," said BITC gender equality director Chloe Chambraud.

"Women in this country want to see the gender pay gap close in their lifetime."

The government said just over 7,350 employers out of an expected 8,900 had reported figures by Thursday afternoon. All companies complied last year, although some needed chasing after missing the deadline.