The pay gap between male and female directors of companies and public bodies has widened, research indicates.
More women are willing to legally challenge pay disparities
On average female executives were paid 22% less than their male counterparts last year, the Institute of Directors (IoD) found.
This figure compared with a 19% pay gap a year earlier.
But for the whole working population, the gender pay gap has shrunk from 17.5% to 17.2%, according to figures from the Office of National Statistics.
The IoD, which analysed pay in 1,250 organisations for its survey, said it was disappointed and urged business to tackle what seemed "an insoluble problem".
The figures mark a setback in efforts to secure equality of pay in the workplace after recent years in which the pay gap has narrowed.
Analysing the pay of about 3,500 directors, the IoD found that the service and voluntary sectors were the worst offenders in terms of pay disparities.
In the service sector, male directors earned an average of £70,657 while their female counterparts were paid £56,933.
Pay differentials were lowest in the public sector, where the average pay gap was 5%.
Female directors of banks and other financial institutions have made advances, with the average pay gap in the financial sector falling to 9% from 14% the year before.
The IoD said the basic pay of female directors had remained broadly unchanged last year while male directors had often enjoyed significant pay rises.
Such discrepancies were hard to explain, it said, and impossible to justify.
"Unless we can achieve equality of opportunity in the near future, we will inevitably face further regulation in this area," said the IoD's director general Miles Templeman.
"The only way to rebut this is for business to act quickly. It is wholly unacceptable in this day and age that it appears that women in comparable positions do not receive the same rewards as their male counterparts."
Recent figures showed a massive rise in equal pay claims being taken to employment tribunals.
The number of claims increased by 155% to more than 44,000 in 2005-2006.
Equality campaigners said more women were willing to challenge their pay because of the "no-win, no-fee" offers from lawyers.