Judges rule that the US constitution gives gay people the right to marry, marking a historic victory for equality campaigners.
The Supreme Court voted 5-4 in favour of the principle that same-sex couples have a constitutional right to marry.
The ruling means all 50 US states will be forced to legalise gay marriage.
President Barack Obama said in a tweet: “Today is a big step in our march toward equality. Gay and lesbian couples now have the right to marry, just like anyone else. #LoveWins.”
Today is a big step in our march toward equality. Gay and lesbian couples now have the right to marry, just like anyone else. #LoveWins
— President Obama (@POTUS) June 26, 2015
Crowds of campaigners waiting outside the court cheered after hearing the news.
Some 13 states currently have bans in place, while Alabama is contesting a court ruling that lifted the ban on same-sex marriage in the state.
Justice Anthony Kennedy, writing on behalf of the court, said that the hope of gay people intending to marry “is not to be condemned to live in loneliness, excluded from one of civilization’s oldest institutions.
“They ask for equal dignity in the eyes of the law. The constitution grants them that right.”
In a dissenting opinion, conservative Justice Antonin Scalia says the court’s decision shows it is a “threat to American democracy.”
The ruling is the Supreme Court’s biggest intervention in marriage rights since 1967, when it struck down a law banning interracial marriage in the state of Virginia.
State laws banning gay sex were only thrown out in 2003, and same-sex marriage was legalised for the first time in 2004 in the state of Massachusetts.
The announcement comes as gay pride parades are set to take place in London, San Francisco, Mexico and other countries around the world marking the anniversary of the police raid on the Stonewall Inn gay club in New York.
Channel 4 News’s Washington Correspondent Kylie Morris writes:“It’s a short hot summer for the US supreme court – with a decision still pending on America’s use of lethal injections to execute death row prisoners.
“But it has now delivered judgements on two key cases that might have seen the court roll back progressive policies on healthcare (Thursday) and same sex marriage (Friday).
“There were tears of relief from passionate supporters of the Affordable Care Act, when a conservative challenge to Obamacare failed. The court ruling keeps in place health insurance for 9 million people, delivered by the 2010 act. It sustains a key achievement of the Obama administration.
“The ruling on same sex marriage will be greeted with jubilation by those Americans wanting to marry same sex partners, but who had been prevented by selective legal arrangements practised mainly in conservative states.
“It also overrules the kind of chicanery practised by district courts in states like Alabama where court officials were encouraged not to issue marriage licences.”