By Lawrence Hurley and Aruna Viswanatha

WASHINGTON, Feb 27 (Reuters) - More than 200 businesses on Wednesday urged the U.S. Supreme Court to strike down a federal law that restricts the definition of marriage to heterosexual unions, in one of corporate America's most prominent efforts to support same-sex marriage.

The companies signed on to a friend-of-the-court brief in Windsor v. United States, a high-profile case challenging the 1996 federal Defense of Marriage Act (DOMA). They ranged from technology giants Microsoft Corp and Google Inc to Wall Street financiers such as Citigroup Inc and Goldman Sachs Group Inc to vineyards and yogurt makers in California.

Thomson Reuters Corp, which owns the Reuters news agency, also supported the submission.

The companies want the Supreme Court to void a key provision in the federal law that defines marriage as a union between a man and a woman. They largely stayed away from constitutional arguments attacking the law and instead focused on the business nuisance the law created.

DOMA forces employers to treat employees with same-sex spouses differently from those with opposite-sex partners, the companies said, depriving gay employees of certain healthcare and retirement benefits that may be on offer. The law also creates headaches for human resources officials, they said.

"HR departments would tell you it is a disaster trying to deal with DOMA when you are a large employer, because you have these employees who are legally married, but now you've got to put them in a different box for W-2s, for ERISA (Employee Retirement Income Security Act), for retirement benefits, and it's really vexing," said Sabin Willett in an interview. Willett wrote the brief for his law firm, Bingham McCutchen, which handled the matter pro bono.

Separately, lawyers representing another group of employers, including some of the same companies, said they planned to file a brief on Thursday in a related case that questions a California law, known as Proposition 8, banning gay marriage.

The two cases are to be argued before the Supreme Court on March 26 and 27. A decision is expected by the end of June.

While corporate America has long offered domestic partnership benefits and made efforts to attract gay employees, the filing seemed to represent a new step in an effort to promote the issue.

"It is old news that big business is friendly to lesbian and gay unions," said Yale law professor William Eskridge, who has argued on behalf of gay rights. "But there has never been a business brief quite like this one with so many signatories on such a landmark issue," he said.

A group of prominent Republicans, including former advisers to President George W. Bush, are also expected to file a brief challenging the California law, adding heft to backers of gay rights.

The arguments appeared directed at Justice Anthony Kennedy, as a moderate and potential swing vote, to show the kind of wide support that exists, Eskridge said.

'HURTING BUSINESS'

The brief grew out of a previous effort to represent business interests in another case challenging the DOMA law, according to Willett.

That case brought together some 70 companies that felt courts may not have understood the full business impact of the law.

"When people talk about DOMA, they usually, and rightly so, focus on its impact upon human beings ... but people may not realize, and courts may not realize, this thing is hurting business, too," Willett said.

In the brief filed on Wednesday, the companies argued that DOMA "requires that employers treat one employee differently from another, when each is married, and each marriage is equally lawful."

DOMA does not create any uniformity nationwide, they said, because 12 states either authorize same-sex marriage or recognize marriages that have been performed in other states.

That creates a burden for employers, particularly those who do business nationally, they added.

The law also forces companies to discriminate, sometimes in contravention of their own internal policies and local laws, when dealing with healthcare plans and other benefits, the companies said.

In briefs already filed in support of restricting marriage to heterosexual unions, business interests have not been represented. The U.S. Chamber of Commerce has not taken a stand on the issue.

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