The US Supreme Court issued a historic ruling on same-sex marriage today, saying gay and lesbian couples in the United States have a constitutional right to marry in all 50 states.

In a 5-4 decision, the highest court in the United States ruled that under the Constitution, states are not allowed to prohibit same-sex marriage.

It also ruled that states must recognise marriage licences legally issued to gays and lesbians by other states.

The two questions, which were argued in April, were brought by gay and lesbian couples in the states of Kentucky, Ohio, Tennessee, and Michigan, who argued against those states' bans on same-sex marriage or their bans on recognising marriage licences issued to gay and lesbian couples by other states.

Marriage in the United States is regulated at the state level.

Thirty-six of the 50 states allow marriage licences to be issued to same-sex couples; the other 14 states banned gay marriage through either legislation or referendum.

Attorneys on both sides of the cases argued whether principles of federalism allow states to define marriage as a matter of public policy, or whether marriage is a fundamental right under the US Constitution.

The states argued that their same-sex marriage bans - created when a majority of voters or legislators defined marriage as between a man and a woman - were a matter of public policy.

The states said they were entitled to define marriage to promote child-bearing and provide a secure environment for children.

They also said that many states were conducting a robust debate on the definition of marriage and argued the Supreme Court would cut off that debate by imposing a uniform national definition of marriage on the country.

The last time the Supreme Court took up a major question on gay marriage was 2013, when it struck down a provision of the 1996 federal Defence of Marriage Act (Doma).

That decision meant same-sex couples who were married in states allowing gay marriage were entitled to federal marriage benefits.

- dpa