Trade union Prospect has written to the equality and human rights commission asking it to investigate the gender pensions gap.
In the letter, Prospect warns that pension inequality is "even more detrimental than the gender pay gap" because it hits women at a time when they have fewer options to do anything about it.
Prospect asked the EHRC to review the extent to which Amber Rudd, secretary of state for work and pensions, has minimised the disadvantages suffered by women as a result of the gender pension gap, arguing the government has missed several chances to help women.
Last year Prospect produced an estimate of the gender pension gap which showed that the overall gender pension gap in the UK in 2016-17 (the latest year data was available for) was 39.5 per cent.
Although the policies in place to tackle the gender pay gap will have an impact on the pension gap there are additional underlying causes, meaning a sizeable gap would remain if the other causes are not addressed, the trade union stated.
The letter stated: "There is no evidence that the Secretary of State is even aware of the level of the gender pension gap.
"Her department does not publish any estimate of its size or analysis of its causes. Without this most basic level of information, it is impossible to begin to tackle the gender pension gap and the disadvantages it causes to women in retirement."
Prospect claimed that Ms Rudd has missed chances to address the gap.
It said the last review of auto-enrolment in 2017 showed that if the earnings trigger was reduced to the level of the lower earnings limit then 1.2m employees would be brought into occupational pension scheme membership and 78 per cent of them would be women.
The letter also stated: "There are known problems with the application of tax relief for low earners who are members of pension schemes that operate ‘net pay’.
"Again this issue disproportionately impacts women but there is no evidence that the Secretary of State has intervened to push HMRC to resolve it."
The letter comes as the EHRC urged the government to act over the unfair treatment of Women Against State Pension Inequality (Waspi), a cohort born in the 1950s who have lost out as a result of changes to the state pension age.