Women earn 14% less than men partly because they do not have enough maths and science qualifications an industry body has said.
Unions say men often earn more than women for the same work
Schools need to offer better career advice and girls must aim for higher paid careers the Confederation of British Industry (CBI) added.
The CBI dismissed recent claims of "ingrained" sexism in UK firms.
However, unions blamed discrimination for the gender pay gap and said firms should be audited to ensure fairness.
The CBI's research forms part of its submission to the government's Women and Work Commission.
In its interim report, published last week, the Commission concluded that discrimination against women in the UK is "deeply ingrained".
WOMEN AT WORK: KEY FACTS
Between 1971 and 2004: Female employment rate rose from 42% to 70%
Women make up only 32% of managers and senior officials
64% of public sector workers are women against 41% in the private sector
Having two children reduces earnings by an average of 10% while three or more children cuts earnings by 15%
Source: Women and Work Commission
The report added that women face "multiple barriers" to career success and the Commission will now consider whether legislation and mandatory pay reviews are necessary to improve opportunities for women.
The CBI has long fought the need for extra pay legislation, arguing instead that people running firms need advice over workplace rights and that state funded childcare ought to be boosted.
In addition, the CBI said schoolgirls needed to study maths and science to appeal to employers.
"The causes of the pay gap are complex...we need more imaginative careers advice for young women so that they set their sights on higher paid jobs," Sue Anderson, CBI human resources policy director, told the BBC.
But unions pointed out that women are often paid less than their male colleagues for doing the same work.
"Employer bodies should be urging their members to audit their workforce so that they can take steps to eliminate this form of discrimination," Brendan Barber, TUC general secretary, told BBC News.