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."\
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Pension experts at Mercer have identified the countries with the best pension systems. At number 10 is Singapore.
The system is based on the Central Provident Fund, which covers everyone in a job. Some of the cash can be withdrawn during your working life, and a prescribed minimum drawn down at retirement as an income.
Overall, Singapore scored 65.9 out of 100. It fared well on sustainability measures, and integrity, but relatively low incomes in retirement dragged its combined score down.
The UK scored 67.6 out of 100. The system was ruled to have great integrity, and good incomes in retirement. The overall scores were also up from the year earlier, as auto-enrolment was rolled out further, bringing more people into workplace schemes.
The researchers, however, were worried about how sustainable the system would be in the future. They called for an increase in minimum pensions, and added that more people ought to be encouraged into workplace schemes and persuaded to contribute more to their pension. They also wanted to see more people saving privately for their pension, and working later in life.
In Chile the state offers means-tested assistance, a mandatory centralised pension for employees to contribute to, and there are voluntary employer schemes.
Chile score 68.2 out of 100. Its highest score was for integrity, with another good mark for sustainability. Relatively low incomes in retirement let it down, and the researchers said the biggest improvements would come from raising the contribution levels.
Canada has a universal flat-rate pension - with a means-tested supplement. There’s an earnings-related pension based on lifetime earnings, plus voluntary workplace and private schemes.
It scored 69.1 out of 100. Its best score was for incomes in retirement, while it also performed well for integrity. Its only relative weak point was how sustainable it might be for the future - particularly because older people don't tend to stay in work.
Sweden has an earnings-related system with notional accounts - although this system was introduced in 1999 so it’s still in transition from a pay-as-you go system to a funded one. There’s also a means-tested top up.
Sweden was given 73.4 out of 100. It scored excellently for integrity, and well for sustainability. The overall score was brought down by incomes in retirement, and the researchers called for more workplace and private pensions.
Switzerland has an earnings-related public pension, a mandatory occupational system and voluntary private pensions.
It scored 73.9 out of 100. It fared well for integrity and reasonably well for incomes in retirement. The researchers just questioned its sustainability.
Finland has a means-tested basic state pension and a range of statutory earnings-related schemes. It scored 74.3 out of 100.
It had high integrity scores, with a less positive result for incomes in retirement, and a surprisingly low score for sustainability. The researchers called for higher minimum pensions, higher mandatory contributions and encouraging people to work longer to improve sustainability.
The Netherlands has a flat-rate public pension and quasi-mandatory earnings-related occupational schemes - which are industry-wide defined benefit schemes based on lifetime average earnings.
The system scored 79.2 out of 100. All its scores were high - particularly for the integrity of the system.
The system in Australia consists of a government scheme, a mandatory employer contribution into a pension, and additional voluntary contributions from individuals.
It benefits from the fact that all workers have been automatically enrolled in their company pension schemes for some time, so participation rates are high. The minimum contributions have also been raised recently, which means workers are building reasonable retirement incomes. It had an overall score of 79.9 out of 100, with the only question mark being over sustainability.
Denmark’s system includes a basic state pension, means-tested state top-ups, a fully funded defined contribution scheme and mandatory occupational schemes.
The researchers said it was "A first class and robust retirement income system". It scored 82.4 out of 100, with high marks across the board.