Theresa May has said Britain must tackle the 'burning injustice' of the gender pay gap as new rules forcing firms to reveal pay data show 78 per cent pay men more.

Mrs May said a failure to pay women as much as men was a problem which damaged society and compared the challenge to women getting the vote 100 years ago.

The Prime Minister hit out as rules came into force that demand all employers with more than 250 staff detail their gender pay gap. As of 8am, nearly all of the 9,000 organisations expected to report their gender pay gap had done so.

Of the 8,874 firms to submit data so far, 78 per cent had a gender pay gap in favour of men, while 14 per cent reported a gap in favour of women. The remaining 8 per cent said they had no gender gap at all.

Theresa May (pictured with husband Philip on Sunday) has said Britain must tackle the 'burning issue' of the gender pay gap as new rules forcing firms to reveal pay data show 78 per cent pay men more.

In research re-published yesterday, economics think tank the Institute for Fiscal Studies said women taking time out of work for child care fuelled the pay gap.

But Mrs May vowed to tackle the underlying causes of unequal pay.

Writing in the Daily Telegraph Mrs May said: 'A hundred years ago, some women first won the right to vote.

'But for all the welcome progress in the decades since, major injustices still hold too many women back.

'When I became Prime Minister, I committed myself to tackling the burning injustices which mar our society. One such is the gender pay gap.'

While the difference in median hourly wages earned by men and women is at an historic low, Mrs May says progress is still too slow and action was needed to close the gap for good within a generation.

'It is essential that we do so. Most importantly, because equality for women is a right, and our whole society is the poorer as long as it remains unrealised,' she said.

The IFS, run by Paul Johnson, said research showed the gender pay gap is fuelled by women being more likely to work part time after starting a family

'There is also a clear economic imperative. It is estimated that if women and men enjoyed parity in their hours, pay and seniority at work then we could see up to £150 billion added to our GDP.'

The IFS data will fuel debate about whether and why there is still a gender pay gap in Britain despite the passing of equalities legislation.

The IFS said its research showed that while there were several factors behind the wage cap, having children is a major factor in why some women's pay falls behind.

It said: 'The research also shows that some, but not all, of the gender wage gap is explained by the fact that after having children women are much more likely than men to work part-time, and part-time work is associated with little or no subsequent progression in hourly wages.

'This is particularly important for graduates for whom full-time experience typically delivers considerable wage growth.'

The IFS praised the Government for demanding firms publish data about hourly earnings rather than annual pay.

This is because annualised figures would exaggerate losses faced by part time workers even if there was little or no difference in hourly pay.

Data published so far on the median gender pay gap reveals 79 per cent of firms pay men more than women

But warning child care was not the only factor, the think tank added: 'The cause of – and therefore the appropriate response by policymakers to – this part of the gender wage gap could be very different to, for example, the part of the gender wage gap that exists prior to the arrival of a first child.'

In some cases a large pay gap is not surprising, it added.

The IFS said: 'For example five organisations are currently reporting that their mean gender wage gap is 80 percentage points or more larger than their median gender wage gap and all five are English Premier League football clubs.'

Companies who do not provide their figures could face legal action including court orders and fines.

Dawn Butler, shadow women and equalities minister, said: 'There is no excuse for companies which fail to meet today's deadline.

'The Equality and Human Rights Commission must use the full force of its powers to ensure employers publish their data or face sanctions.'

The gender pay gap is calculated as the difference between the average salaries of men and women - it is not the same as equal pay, where firms are required to pay people doing the same job the same salary regardless of gender.

The national median gender pay gap is 18.4 per cent.