The Supreme Court has ruled the US Constitution provides same-sex couples the right to marry in a historic triumph for the American gay rights movement.

The court ruled 5-4 that the Constitution’s guarantees of due process and equal protection under the law mean that states cannot ban same-sex marriages.

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With the ruling, gay marriage will become legal in all 50 states.

Justice Anthony Kennedy, writing on behalf of the court, said 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,” the statement said.

Justice Kennedy, a conservative who often casts the deciding vote in close cases, was joined in the majority by the court’s four liberal justices.

There are currently 13 state bans in place, while another state, Alabama, has contested a court ruling that lifted the ban there.

President Barack Obama, in heartfelt remarks, praised the ruling as “a victory for America”.

“Today we can say, in no uncertain terms, that we’ve made our union a little more perfect,” he said at the White House, which changed its Twitter avatar to the rainbow colours of the growing gay rights movement.

“This decision affirms what millions of Americans already believe in their hearts: when all Americans are treated as equal, we are all more free.”

Gay rights supporters celebrate the US Supreme Court ruling.

The ruling is the Supreme Court’s most important expansion of marriage rights in the US since its landmark 1967 ruling in the case Loving vs Virginia that struck down state laws barring interracial marriages.

In a dissenting opinion, conservative Justice Antonin Scalia said the court’s decision was a “threat to American democracy”.

“[The ruling] says that my ruler, and the ruler of 320 million Americans coast-to-coast, is a majority of the nine lawyers on the Supreme Court,” he said.

Conservative Chief Justice John Roberts also read a summary of his dissenting opinion from the bench.

The decision follows rapid changes in attitudes and policies toward gay marriage in America. It was not until 2003 that the Supreme Court threw out state laws banning gay sex.

In 2004, Massachusetts became the first state to legalise same-sex marriage.

Gay marriage has gained increasing acceptance in opinion polls in recent years, particularly among younger Americans.

Same-sex marriage supporters celebrate court victory

Flag-waving LGBT rights advocates on the packed Supreme Court forecourt — some in tears — cheered, danced and sang The Star-Spangled Banner in celebration.

Prominent in the crowd was Jim Obergefell, the lead plaintiff in the case, clutching a photo of his deceased husband John.

Plaintiff Jim Obergefell holds a photo of his late husband John Arthur.

A clearly affected Mr Obergefell took a brief phone call from Mr Obama, who told him: “Not only have you been a great example for people but you’re also going to bring about a lasting change in this country.”

“And it’s pretty rare where that happens so I couldn’t be prouder of you and your husband. God bless you.”

Mr Obergefell, who was live on television at the time, replied humbly: “Thank you, sir. That means an incredible amount to me.”

Robert Westover, 52, and Tom Fulton, 57, hugged and kissed at the news.

“It feels like my birthday, the prom, our wedding day. It’s hard to express the intensity of this moment, that our love now is equal,” said Mr Westover.

United Nations secretary-general Ban Ki-moon welcomed the ruling as “a great step forward for human rights.”

Gay marriage gaining acceptance in Western countries

Last month in Ireland, voters backed same-sex marriage by a landslide in a referendum that marked a dramatic social shift in the traditionally Roman Catholic country.

Ireland followed several Western European countries, including Britain, France and Spain in allowing gay marriage, which is also legal in South Africa, Brazil and Canada. But homosexuality remains taboo and often illegal in many parts of Africa and Asia.

The Supreme Court’s ruling came in a consolidated case, pulling together challenges filed by same-sex couples to gay marriage bans in Kentucky, Michigan, Ohio and Tennessee.

Same-sex marriage was legal in 36 states and Washington, DC. In a 37th, Alabama, a federal court struck down the gay-marriage ban, but the state supreme court has stopped local officials from issuing marriage licenses to gay couples.

But opponents say same-sex marriage legality should be decided by states, not judges.

Republican presidential candidate Mike Huckabee said: “This flawed, failed decision is an out-of-control act of unconstitutional judicial tyranny.”

Rick Santorum, also a Republican presidential candidate, lamented that five “unelected judges redefined the foundational unit of society”.

Some opponents argue it is an affront to traditional marriage between a man and a woman and the Bible condemns homosexuality.

The emotions of the issue were apparent during the court’s April 28 oral arguments in the case when a protester in the courtroom shouted at the justices that they would “burn in hell” if they backed gay marriage.

Mr Obama is the first sitting president to back gay marriage and his administration argued on the side of the same-sex marriage advocates.

The legal repercussions for same-sex couples are broad, affecting not just their right to marry but also their right to be recognised as a spouse or parent on birth and death certificates and other legal papers.

Big business had urged the justices to support gay marriage, saying in a brief submitted in the case that inconsistent state laws impose burdens on companies and marriage bans can conflict with corporate anti-discrimination and diversity policies.

The ruling is the latest milestone in the gay rights movement in recent years.

In 2010, president Barack Obama signed a law allowing gays to serve openly in the US military.

In 2013, the high court ruled unconstitutional a 1996 US law that declared for the purposes of federal benefits marriage was defined as between one man and one woman.