A study has said gay couples in the US can pay thousands of dollars more in tax each year than similar straight couples, because the federal government does not allow married gay couples to file tax returns together.

The disparity in the figures calculated during the joint study by CNN Money and H&R Block are one result of the Defense of Marriage Act.

The heavily-debated Act defines marriage on the federal level as being between a man and a woman, so while individual states can draw up tax systems according to their own laws on marriage equality, gay couples must file their federal tax returns individually.

The study examined a range of scenarios and determined that for couples with children, one of whom stays at home, one of whom earns more than $100,000 each year, the increased cost for a gay couple was mainly a result of larger exposure to income tax: $4,500.

They added that the way tax credits for children, worth $1,000 per child, were phased out more quickly for couples classed as unmarried by the federal government, which brought the increased cost up to over $6,000 a year.

CNN said other factors which leave gays out of pocket include inheritance tax rules and higher capital gains tax exposure for property ownership.

The analysis also pointed out that with a more complicated tax situation, gays may well spend more on tax accountants’ fees than equivalent straight couples.

Differences in the way gay and straight couples are treated for tax purposes also causes difficulties in Europe.

In November, an EU conference heard that disparities in how countries regard foreign marriages and civil partnerships mean gay couples are facing particular struggles to move freely in the European Union.

The higher costs of living as a gay couple in some member states, caused by inheritance, pensions and tax differences similar to those encountered in the US, are seen as a barrier to movement within the EU.