Complex rules mean unmarried couples could miss out on pensions when their partner dies.
House of Commons library figures show only six in 100 members of private schemes can pass on pensions to a partner as of right, while eight in 10 must rely on the discretion of trustees.
It means up to a million couples are living without any future financial guarantees if one of them dies.
Now MPs are to investigate whether people living together should have the same pension rights as those who are married or in civil partnerships.
The Commons Women and Equalities Committee is to take evidence from retired BT project manager Ray Usher, 69.
He has spent a year trying to ensure his partner of 20 years, Linda Drummie, 68, is provided for in the event of his death.
Ray has nominated the former GP’s receptionist for the half share of his occupational pension she would be entitled to if she was his widow.
And although BT pension providers have not said no, they have not said yes either – and warn it could take years to sort out.
They told Ray: “The experience of a surviving partner is very different to the experience of a surviving spouse.”
Ray, of Whitton, South West London, said: “Everyone tells us we should have nothing to worry about but no one will put that in writing.
“It leaves too much uncertainty. We are being treated like third class citizens.”
The couple own their home, have joint bank accounts and utility bills are in both names.
If they were married, Linda would simply have to produce a death certificate.
But as they are not she must fill out forms after Ray’s death proving financial dependency for pension trustees to consider.
And what unmarried partners may get is governed by different rules in different pension schemes.
Former pensions minister Steve Webb said: “I strongly suspect Ray would have his wishes respected. But I can see why they don’t like the element of discretion and uncertainty.”