Pensions Minister Steve Webb will have to explain why the government is continuing to allow same-sex couples to be treated differently from heterosexuals in a select committee appearance later this month.

The Liberal Democrat MP is due before the Pensions Select Committee on 23 October.

It’s expected Mr Webb will answer questions on occupational pension rights for civil partners and same-sex married couples.

Currently, the Marriage (Same Sex Couples) Act does not require schemes to provide same-sex spouses with the same survivor benefits as heterosexual married couples.

The Equality Act 2010 contains a loophole allowing for private occupational pension schemes to ignore years of contributions by gay employees and limit survivor benefits for civil partners.

A succession of MPs and peers from all sides of the political divide pressed the case for the same-sex marriage act to close the loophole ahead of Royal Assent.

But the government resisted the demand and the Department for Work and Pensions (DWP) warned that the cost of equalising pensions would be £18 million.

In July, Conservative MP Mike Freer said it was right to settle the discrepancy “whatever the cost”. He also disputed the DWP’s estimate.

The month before, the Director of Liberty, Shami Chakrabarti, said: “The government argues that equalising pensions would leave providers facing previously unforeseen costs. But pension provision by its very nature is speculative. No provider can be sure how many individuals in a pension scheme will be gay – let alone the number who might marry or form a civil partnership with a partner who happens to outlive them significantly. Providers are constantly required to deal with the uncertainties of life – from the possibility of illness to the decision to get married. Besides, cost is no excuse for blatantly discriminatory measures.”