* HSBC says men paid 59 percent more than women on average
* Companies have until Wednesday to submit gender pay figures
LONDON, April 4 (Reuters) - HSBC, Europe's largest bank, has reported the largest difference in male and female staff salaries among large organisations in Britain, as the government looks to pressure big companies to reduce gender pay disparities.
Swamped by the task of delivering Brexit, Prime Minister Theresa May is under pressure to make visible progress on a domestic agenda set out when she took power in July 2016, particularly after a snap election in 2017 that exposed a weakness on social reforms.
Hoping to highlight gender discrimination and force companies into action, May has implemented long-planned reforms ordering companies with 250 or more employees to publish details of the salary difference between male and female employees by Wednesday evening and report back annually on the pay gap.
HSBC on average paid men 59 percent more than women, the biggest difference among companies with more than 5,000 employees in Britain, according a Reuters analysis of the published data using the mean as the measure.
The next largest gender pay gaps were at Virgin Atlantic, where men received 58 percent more on average more than women, followed by a unit of Barclays, where female staff earned 48 percent less than male colleagues.
HSBC said it was confident in its approach to pay and made appropriate adjustments if it identified differences between men and women in similar roles which could not be explained by performance or experience.
Virgin Atlantic said the gap reflected the relatively low number of female pilots in the aviation industry, while Barclays said it had more work to do so women could progress in their careers at the company.
Firms are not required to break down the data in detail, leading to criticism that the average figures could obscure or exaggerate demographic explanations for disparities. Nevertheless, they offer a step forward in assessing the issue.
Almost 50 years since the passage of Britain's equal pay act, the continued gulf in earnings between men and women has steadily risen up the political agenda. The opposition Labour Party first created powers on gender pay reporting in 2010, but then lost an election. May enacted those powers last year.
Although part of a broader trend of pro-equality policies in Britain, the issue has gained momentum under May, the country's second ever female leader, and increasing competition from her main opponent, socialist campaigner Jeremy Corbyn.
May vowed in 2016 to tackle "burning injustices" in society, with a specific reference to gender pay alongside issues like race and class discrimination.
Nearly two years later, she said that by introducing the reporting requirement, her government had taken a lead on the issue.
"By making this information public, organisations will no longer have anywhere to hide," she wrote in the Daily Telegraph newspaper. "Shareholders and customers will expect to see improvements, and will be able to hold organisations to account if they fail to achieve them."
Other countries to introduce mandatory gender pay gap reporting include Australia, which passed similar legislation in 2012, and Germany.
Reuters analysed pay figures for 491 of the leading companies, government departments, charities, local authorities and hospital trusts, which employ more than 5,000 people.
Of those organisations that had published data by 3 p.m. (1400 GMT) on Wednesday, 97 percent pay men more than women and just 3 percent pay women the most.
The average gender pay gap among these largest companies is 15.5 percent, according to the Reuters analysis.
Political opponents, including Labour, welcomed publication of the data as a step forward but urged a more hands-on approach, saying the government still lacked clear plans on how to address the problems they had revealed.
"(Theresa May) wants to close the #paygap but says nowhere how she plans to do it. Politicians can't just keep beating up business while leaving structural inequality untouched," tweeted Sophie Walker, leader of the Women's Equality Party. (Editing by Guy Faulconbridge and David Holmes)