The lawyers for two elderly sisters are due in court to begin their attempt to win the same inheritance tax rights as married, gay and lesbian couples.
The Burden sisters say they are being discriminated against
Joyce and Sybil Burden, from Wiltshire, have lived together since birth but if one of them dies the other will face a large inheritance tax bill.
They fear the one left will have to sell the family home to pay it.
Joyce, who is 88, and Sybil, who is 80, are taking the case to the European Court of Human Rights.
They decided to take action in the wake of the Civil Partnership Act 2004.
It granted the same rights that married couples enjoy to gay and lesbian couples, but not to family members.
If a husband or wife dies then the estate passes to the surviving spouse without paying tax.
The Burden sisters say this amounts to discrimination under the terms of the European Convention of Human Rights.
Christopher Tidmarsh QC, an inheritance law expert, said while the European Court must think the case had some merit, the sisters would face a tough fight.
"I think it is a very difficult case myself.
"The difficulties lie in actually describing what it is about the relationship that brings it within the exemption or makes it analogous to the exemption."
Ahead of the case at the European Court of Human Rights in Strasbourg, the government refused to comment.
But a spokesman said inheritance tax affected only 6% of estates.
Ann Widdecombe, the Conservative MP, said changes to the current system are needed.
"You would get rid of all these anomalies if you just get rid of the tax or else raised the threshold to such a point that you'd really be wealthy enough to sustain it."
Inheritance tax affects estates which are valued at more than £285,000 - but the government intends to further increase the threshold to £325,000 by 2010.
An estate includes the value of a person's property and as house prices have increased dramatically in recent years, the property can easily make up the bulk of the £285,000 threshold.
This situation is most acute if the deceased person lived in London or a similar property hot spot.
Some tax experts believe that the threshold could be increased to £400,000 - which would mean only the very wealthy would fall within the tax.