The US Supreme Court has ruled that juries in state criminal trials have to reach a unanimous decision to convict a defendant.

The 5-4 ruling on Monday smooths out a legal bump that allowed for divided juries in Louisiana and Oregon to convict defendants in criminal trials.

Justice Neil Gorsuch wrote the opinion on the decision, and was joined partially by Justices Stephen Breyer, Ruth Bader Ginsburg, Stepehen Breyer, Sonia Sotomayor and Brett Kavanaugh. Justice Clarence Thomas concurred in the judgement and Justice Samuel Alito dissented, joined partially by Justice Elena Kagan and fully by Justice John Roberts.

“This court has repeatedly and over many years recognised that the Sixth Amendment requires unanimity,” Mr Gorsuch wrote in the opinion.

In his dissent, Mr Alito warned that the ruling would “impose a potentially crushing burden on the courts and criminal justice systems” in Louisiana and Oregon.

According to the Associated Press, the Supreme Court’s decision overturned a 1972 Supreme Court precedent and means 2016 murder conviction of Evangelisto Ramos, a man serving a life sentence in Louisiana for the killing of a woman. He was convicted after a jury voted 10 to 2 to convict him.

Mr Ramos was sentenced to life in prison without parole for the murder of Trinece Fedison. The woman’s body was found in a trash can in New Orleans.

Louisiana overturned its law allowing for convictions by non-unanimous juries in 2019, but the ruling did not retroactively apply to Mr Ramos’s case.

“We are heartened that the court has held, once and for all, that the promise of the Sixth Amendment fully applies in Louisiana, rejecting any concept of second-class justice,” Mr Ramos’s lawyer, Ben Cohen, told Reuters.

The Supreme Court’s precedent establishes the nationwide precedent that a jury must reach a unanimous decision to convict a defendant.

People with criminal cases who are appealing their convictions will likely be affected by the ruling. Those whose criminal cases have been concluded and are not in the appeal process will likely remain unaffected unless further lawsuits establish that the new ruling applies retroactively.