In a major policy reversal, the Obama administration has said it will no longer defend the constitutionality of a US law banning recognition of same-sex marriage.
Attorney General Eric Holder said President Barack Obama has concluded that the administration cannot defend the federal law that defines marriage as only between a man and a woman.
He noted that the congressional debate during passage of the Defence of Marriage Act "contains numerous expressions reflecting moral disapproval of gays and lesbians and their intimate and family relationships - precisely the kind of stereotype-based thinking and animus" the US constitution is designed to guard against.
The Justice Department had defended the act in court until now.
The move quickly drew praise from some Democrats in Congress but a sharp response from the spokesman for Republican John Boehner, the leader of the House of Representatives.
"While Americans want Washington to focus on creating jobs and cutting spending, the president will have to explain why he thinks now is the appropriate time to stir up a controversial issue that sharply divides the nation," said Mr Boehner's spokesman Michael Steel.
Gay groups, which had long pressured the administration to take a step like this, were pleased.
Rea Carey, executive director of the National Gay and Lesbian Task Force, called the policy change "a tremendous step toward recognising our common humanity and ending an egregious injustice against thousands of loving, committed couples who simply want the protections, rights and responsibilities afforded other married couples. We thank the Obama administration".
Mr Obama's move may position him politically at the forefront of rising public support for gay marriage.
Polling results can vary rather significantly depending on what words are used to describe gay marriage, but there is a gradual trend in public opinion toward more acceptance of gay marriage.