Two elderly sisters who have lived together since birth will take the government to a European court on Tuesday to seek the same exemption from death taxes as married and gay couples.
Joyce and Sybil Burden fear that if one of them dies the other will have to sell their jointly owned home to pay inheritance tax.
Under current laws, married or same sex couples would inherit a property tax free if their partner died.
Joyce Burden, 88, and Sybil, 80, say co-habiting siblings should have the same rights. They have taken their fight to the European Court of Human Rights in Strasbourg.
We are looked down upon for being single, Sybil Burden told The Times. We just want to be treated as equal citizens and given the rights we deserve.
Inheritance tax is levied at 40 percent on all assets above a 285,000 pounds threshold. Rising house prices mean more people are paying the tax.
The Burdens live on the farm estate where they grew up near the town of Marlborough in Wiltshire. Media reports say their estate is worth 875,000 pounds.
They have written to the Chancellor of the Exchequer every year since 1976 to lobby for a change in the law and turned to the European Court of Human Rights in March 2005.
They are just hoping we die before we get to court, Joyce Burden told the Times. But they don't know how determined we are to see this through.
A government spokeswoman declined to comment on the hearing.
The number of estates paying inheritance tax rose by 72 percent to 30,451 in the five years to 2003 to 2004, Britain's biggest mortgage lender Halifax said in a report last month.
It said house prices have risen by 179 percent in the past 10 years, compared with just an 85 percent increase in the inheritance tax threshold.
Former Labour minister Stephen Byers called for the tax's abolition last month, calling it a penalty on hard work, thrift and enterprise.