LONDON: British women are effectively working for free from now until the end of the year because of the lingering "pay gap" between what they and male colleagues earn, researchers said on Monday.

Women working full-time take home about $7,500 less a year than men, according to a report by the Fawcett Society, a campaign group promoting women's rights in the labour market.

This year's Equal Pay Day, which marks the day after which women "work for free" due to the pay gap, comes 52 days before the end of the year compared to 56 last year, it said.

The gap between male and female earnings stands at 14.2 per cent according to the research, an improvement on 15.7 per cent last year.

Just over half of British women said equal pay was their biggest concern in the workplace in a major poll published last month by the Thomson Reuters Foundation and the Rockefeller Foundation.

It was also cited as the biggest concern by women in the United States, France, Canada, Brazil and Australia in the poll of more than 9,500 women in the G20 nations conducted by international pollster Ipsos MORI.

"There has never been a better opportunity to close the pay gap for good," said Fawcett Society chief executive Sam Smethers.

"We could speed up progress towards the day when we can consign it to history."

British Prime Minister David Cameron, who has called the gap "a scandal," has pledged to force large companies to publish the average pay of male and female employees.

"It is time to have the conversation and ask your employer if they are ready for the new pay gap reporting requirements," said Smethers in a statement.

The Fawcett Society also said employers should promote flexible working arrangements, and that the government should invest more in childcare.

In 2014, the last year for which figures were available, Britain went from 18th place to 26th in the World Economic Forum's annual Global Gender Gap Report.

Iceland, Finland, Norway, Sweden and Denmark were ranked the most gender-equal societies in the world, with Yemen worst.