Two sisters in their eighties who claim that the law treats them less favourably than if they were a lesbian couple are to challenge Britain’s inheritance tax rules in Europe today.

Joyce Burden, 89, and her sister Sybil, 81, fear that they will have to sell the home they jointly own in Marlborough, Wiltshire, to pay inheritance tax when the first of them dies.

They will make a last-ditch appeal before 17 Grand Chamber judges at the European Court of Human Rights in Strasbourg, saying that they should have the same legal rights as gay couples in civil partnerships. An ordinary chamber of seven judges ruled against them last December by four to three.

Inheritance tax is levied at 40 per cent of the value of the estate, after an exemption is made for the first £300,000. Husbands and wives are exempt from the tax on each other’s estate; and, since the Civil Partnership Act 2004, so are gay couples.

The sisters’ house, which cost £7,000 to build in 1965, was valued last year at £875,000, attracting inheritance tax of about £236,000.

The sisters, who have lived together all their lives, argue that they should be treated no less favourably than civil partners. Because both are elderly, their estates are likely to be taxed twice within a few years: once on the first death, and again on the second.

The Government says that favourable tax treatment for civil partners is justified, because it encourages committed relationships. Ireland and Belgium have filed arguments with the court in support of the UK’s position.

When the Civil Partnership Bill was going through Parliament, the House of Lords tried unsuccessfully to extend benefits to family members aged over 30, who had lived together for at least 12 years. The European Court ruling last December held that the Government was justified in giving tax benefits to some groups while excluding others, to achieve legitimate social policy aims.

Julian Washington, a partner at Forsters, the Mayfair law firm, said that a victory by the sisters would force a “major revision to the UK tax system”.

Describing the battle as being like David against Goliath, he said: “It would be unwise to underestimate Joyce and Sybil Burden. It is impossible not to be impressed by their tenacity. These octogenarian spinster sisters argue that it is quite simply unfair that, when one of them dies, the survivor may have to sell their shared home to pay the inheritance tax.

“By contrast, they point out that if they were a lesbian couple in a civil partnership, they could benefit from complete tax exemption.”

But Julia Whittle, principal with Punter Southall, the nationwide financial adviser, said that a victory could cause a difficult precedent for the Revenue because the bulk of a family estate was often tied up in property. She said that if the judgment went against the sisters they could approach the Revenue for special consideration on how to pay the tax bill.

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