Two elderly sisters fighting for the same rights as married and gay couples have lost a last-ditch legal appeal for equal treatment.

In a 15-2 vote, the European Court of Human Rights in Strasbourg ruled that Joyce and Sybil Burden, who have lived together all their lives, do not face unfair discrimination under UK inheritance tax rules.

Joyce, 90, and 82-year-old Sybil have been fighting for decades to avoid crippling inheritance tax on their home in Marlborough, Wiltshire, when one of them dies.

They claimed UK inheritance tax laws breached their human rights by exempting married and gay couples from paying inheritance tax, while targeting cohabiting siblings.

But the Grand Chamber of the human rights court upheld an earlier ruling that national governments were entitled to some discretion when deciding taxation arrangements.

The decision, a major blow to the sisters, means that when one of them dies the other will have to sell their four-bedroom property to pay the 40% inheritance tax on its value above £300,000.

If they had won their case, UK inheritance tax law would have had to change, to place cohabiting couples on an equal footing with married couples and "civil partnerships" in being exempt from inheritance tax.

The sisters have been fighting the battle for decades - writing to the Chancellor of the day before every Budget since 1976.

And when the UK Civil Partnership Act of 2004 first recognised gay and lesbian couples for inheritance tax purposes, the sisters turned to the European Court of Human Rights, claiming the Act violated Human Rights Convention articles outlawing discrimination and guaranteeing the "protection of property".

In 2006 the Burdens lost the case by a 4-3 majority of the panel of seven human rights judges - although three members of the court described their plight as "awful".

But the appeal hearing, before a larger 17-member panel of human rights judges, produced a more decisive 15-2 majority against the sisters yesterday.

The ruling marks the end of the road for the sisters' legal bid. After losing the first case in 2006, Joyce Burden commented: "If we were lesbians we would have all the rights in the world. But we are sisters, and it seems we have no rights at all."

Yesterday's ruling pointed out that relationships between siblings were considered to be "of a different nature" to those between married couples and gay civil partners under the UK Civil Partnership Act.

One of the defining characteristics of a marriage or civil union under the act was that it was forbidden to close family members - and the fact that the Burdens had chosen to live together all their adult lives did not alter the "essential difference" between the two types of relationship.

When the Act came into force, gay couples had the choice to enter into a legal relationship designed by parliament to correspond as far as possible to marriage.

The judges said the legal consequences of such civil partnerships, just like those of marriages, set such relationships apart from other forms of cohabitation.

The Burden sisters vowed last night to continue lobbying parliament on the issue despite their "bitter disappointment".