Caroline Lucas, the Green MP for Brighton Pavilion, has tabled an amendment to the Pensions Bill, which would give equal provision of survivor benefits for same-sex couples.
The Commons has been debating the Pensions Bill this week, and it will enter its committee stage on Thursday.
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.
In recent months several MPs in the Commons and peers in the House of Lords have attempted to address this discrepancy by tabling amendments to the Marriage (Same Sex Couples) Bill – which in its current form will continue the inequality.
The effect of a successful amendment would be to remove discrimination for all couples, whether or not a member of a couple has undergone gender reassignment and whether a couple is gay or straight.
Labour peer Lord Alli has tabled a new amendment to the same-sex marriage bill, which is identical to that of the Ms Lucas.
The government has said the cost of closing the loop hole would be £18 million.
Detailed line by line examination of the bill will continue by peers. Third reading takes place on 15 July.