A gay man is fighting to win his husband the same pension rights a wife would enjoy if he was in a heterosexual relationship.
Old Harrovian and former cavalry officer John Walker has launched an equal treatment case in the Court of Appeal which could have widespread repercussions.
The Government estimates that "full equalisation" would cost around £3.3bn and have complex implications for pension schemes.
Mr Walker, 62, retired from chemicals group Innospec Ltd in 2003 after 23 years' employment.
He began his relationship with his partner, a 49-year old former computer executive, in 1983.
They entered into a civil partnership in January 2006, which was recently converted into a marriage.
Mr Walker wants to ensure that, should he die first, his husband will be adequately provided for.
His legal action is against Innospec which, he argues, unlawfully fails to treat surviving same-sex spouses and civil partners the same as the widows or widowers in heterosexual marriages.
His lawyers say in written submissions before three judges that, if Mr Walker were to dissolve his same-sex partnership and marry a woman, she would be entitled in the event of his death to approximately £41,000 a year.
But the trustees of the Innospec pension scheme would only pay his husband around 1% of that amount.
An Employment Tribunal in Manchester ruled in 2012 that the Innospec scheme contravened both European Union law and the European Convention on Human Rights.
But last year Innospec appealed to the Employment Appeal Tribunal (EAT) with support from the Department for Work and Pensions (DWP).
Allowing the appeal, the EAT ruled an exemption in the Equality Act 2010 disapplied pension rights accrued by Mr Walker before December 5 2005 - the date when the Civil Partnership Act 2004 came into force and required benefits to be provided equally to civil partners and married couples.
The EAT held those rights did not have retrospective effect or allow inequalities in pay based on sexual orientation prior to that date to be addressed.
Mr Walker is asking the appeal judges - Lord Dyson, Master of the Rolls, sitting with Lord Justice Lewison and Lord Justice Underhill - to rule the tribunal decision flawed as it amounts to direct discrimination and a breach of human rights laws.
He also argues it is contrary to an EU directive that sets out the framework for equal treatment in employment.
A DWP spokesman said: "It is right all workers are now treated equally and survivor benefits are built up on an equal basis.
"But full equalisation would cost around £3.3bn and the implications for pension schemes are complex.
"We must consider the full impact of this issue before considering changes to legislation."\