A gay man fighting to win his husband the same pension rights a wife would enjoy if he was in a heterosexual relationship takes his case to the UK's highest court today.
Ex-cavalry officer John Walker is hoping that five justices at the Supreme Court in London will overturn an earlier ruling against him.
Human rights organisation Liberty, which is representing Mr Walker, says a successful outcome could "dramatically change the lives of thousands of same-sex couples".
He suffered a defeat at the Court of Appeal in 2015, when judges ruled that his claim failed because it applied to a period before gay civil partnerships were recognised by the law.
Mr Walker retired from chemicals group Innospec Ltd in 2003, after working for the company for more than 20 years. He had made the same contributions to the pension scheme as his heterosexual colleagues.
He and his husband, a former computer executive, have been together since 1993.
The Civil Partnership Act 2004 came into force on December 5 2005, and Mr Walker and his partner entered into a civil partnership in January 2006, which was later converted into a marriage.
Liberty said the case "challenges an exemption in the Equality Act that lets employers exclude same-sex partners from spousal benefits paid into a pension fund before December 2005, when civil partnerships became legal".
It will be argued that the exemption is "discriminatory".
Mr Walker, 65, wants to ensure that, should he die first, his husband, 52, will be adequately provided for.
Liberty points out that his husband would receive a pension of only a few hundred pounds a year - but if he was married to a woman she would be entitled to receive around £45,000 a year for life.
Mr Walker said: "The Government should be ashamed that, in 2017, I and so many others are being forced to live with the worry that our loved ones won't be provided for when we're gone, solely because of our sexuality.
"My husband and I have been together for 24 years. During that time, I also gave more than two decades of my life to Innospec, paying in exactly the same amount into the company pension fund as my heterosexual colleagues.
"How can it be right that my husband will get practically nothing but, if I were to divorce him and marry the very first woman I see, she would be immediately entitled to the full spousal pension? It's not just unfair - it's absurd."
Liberty lawyer Emma Norton said "many, many others will be suffering the same injustice", adding: "We hope the Supreme Court will drive the law into the 21st century and take a huge step towards equal pension rights for same-sex spouses and civil partners."