The gender pension gap is defined as the difference in average gross pension income for women and men receiving state pension, according to trade union Prospect.
The figure has dropped slightly since 2014/15, when it was 41.6%.
“Gender-based economic inequality may start in the workplace, but it follows women for the rest of their lives,” said Sue Ferns, Prospect’s senior deputy general secretary.
“It is not acceptable that women are condemned to less comfortable retirements and greater anxiety about finances because of inherent unfairness in the labour market and structural problems in the pension system,” she added.
The government does not publish an official estimate of the gap, therefore the figures come from analysis of the Department for Work and Pensions family resources survey.
Ferns said the government should have a duty to produce an annual report on the gap to focus attention and help to tackle it.
“There are also practical steps that government must take such as abolishing the automatic enrolment earnings trigger which disproportionately excludes women from occupational pension scheme membership,” she said.
The analysis found that the gender pensions gap is more than twice that of the gender pay gap, which according to the latest Office for National Statistics (ONS) figures released last week is now 17.9% for all employees.
The ONS figures also showed that the gender pay gap for full time employees had dropped 0.5% to 8.6% since last year for full time employees, however, for women over 60 it had increased 12.9% to 15.6% in the same period.