The US state of Massachusetts, which has legalised same-sex marriage, is suing the federal government over the Defence of Marriage Act.

The lawsuit, brought by the first state to legalise gay marriage, claims that DOMA infringes on a state’s sovereign right to define marital status. They are seeking to gain a ruling that DOMA is unconstitutional.

DOMA prohibits the federal government from extending benefits that are tied to marital status — such as spousal Social Security benefits and survivor rights to federal pensions — to same-sex couples, even if they are legally married in the six states that allow such weddings.

The complaint says DOMA prevents MassHealth, the state’s insurance program for low-income residents, from covering gay couples as married — forcing the state to ignore its own laws — because it is partly funded by federal dollars.

In a lawsuit filed in U.S. District Court here, Massachusetts — which legalised gay marriage in 2004 — said that in enacting DOMA, Congress “overstepped its authority, undermined states’ efforts to recognise marriages between same-sex couples, and codified an animus toward gay and lesbian people.”

The Department of Justice declined to comment pending a review of the case, but a spokesman for the agency noted that President Barack Obama supports the legislative repeal of DOMA.

Nothing in the U.S. Constitution obligates Washington to follow a state’s definition of marriage.

Same-sex marriage became an exception to this rule as the federal government tried to protect the traditional definition of marriage by passing DOMA.

The new administration’s Justice Department defended DOMA in court in a case in which a same couple in California sought federal marital benefits.

In a filing in the case last month, the government said DOMA “continues the longstanding federal policy of affording federal benefits and privileges on the basis of a centuries-old form of marriage, without committing the federal government to devote scarce resources to newer versions of the institution that any State may choose to recognise.”

In March, a Massachusetts group of gay couples and gay widowers who had been legally wed sued the federal government, claiming discrimination over benefits. The Department of Justice declined to comment and hasn’t yet responded in court.