A gay man is going to the Supreme Court to call for his husband to get the same pension benefits that a wife would get.
John Walker, 65, is fighting what he calls an "absurd" legal exemption which means that, in the event of his death, his partner is entitled to survivor benefits only from December 2005.
However, if he had a wife she would be entitled to half of his pension, based on when he began paying in.
Liberty says the court's decision could influence "the lives of thousands".
The majority of occupational pension schemes give 50% of the value of a pension to a spouse for the rest of their lives after their husband or wife dies, without taking the marriage date into account.
The Equality Act 2010 has an exemption for employers, allowing them to exclude civil partners from spousal benefits paid in before 5 December 2005 - the date civil partnerships became legal.
According to campaign group Liberty, the majority of companies have ignored the exemption and extended the same rights to those in gay relationships.
However, Innospec Ltd, based in Cheshire, where Mr Walker worked for more than 20 years and accrued a pension of £85,000 a year, has not.
His husband would be entitled to about £500 per year under the current exemption. This figure would rise to £45,000 if he had married a woman.
The BBC has contacted the chemicals company for comment.
"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," said Mr Walker, who has been with his partner for 24 years and entered a civil partnership in 2006, later converting it to a marriage.
"I 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."
Mr Walker won an employment tribunal in 2012 to secure the pension rights, but it was overturned on appeal by Innospec two years later and that decision was upheld by the Court of Appeal in 2015.
On Wednesday, he began a two day hearing at the Supreme Court in a last attempt to change the rules.
'No price tag on equality'
Emma Norton, lawyer for Liberty, who is representing Mr Walker, said it was a "clear case of discrimination".
"This archaic loophole has no place in the UK in 2017, and it is disgraceful that the Department for Work and Pensions continues to spend taxpayers' money fighting to preserve it," she said. "There can be no price tag on equality."
"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."
The Department for Work and Pensions said it was aware of the case.
A date for the verdict from the court has not been announced.