The Supreme Court ruled yesterday that a trial on the constitutionality of California’s gay marriage ban cannot be broadcast on the internet.

Justices had placed a temporary three-day ban on making the trial public on Monday, saying they needed more time to consider the issue.

Yesterday, they decided 5-4 that the trial would not be broadcast before it concludes.

Supporters of the gay marriage ban had said they could be targeted if the case was broadcast and cited claims of death threats.

In the ruling, justices agreed that they could be subject to intimidation and “irreparable harm”.

The decision was split down liberal and conservative lines, with the four liberal justices voting for the broadcasting.

One, Justice Stephen G Breyer, called the decision “unusual” and said there was no precedent for blocking public access to proceedings.

He wrote: “It identifies no real harm, let alone ‘irreparable harm’. . . . And the public interest weighs in favour of providing access to the courts.”

Breyer added that all those who would be appearing to argue for the gay marriage ban had made extensive public and television appearances in the last year.

Judge Vaughn Walker had made the decision to allow broadcasting of the trial, saying it was of great importance to the public.

The Supreme Court criticised him for making the decision at a late stage.

Rick Jacobs, chair of the gay equality group Courage Campaign, said: “The Supreme Court just struck a huge blow against transparency and accountability.

“The five conservative justices are enabling the Prop 8 supporters to mask their radical views, preventing the transparency that an effective court system needs.

“This historic trial will remain largely hidden from public view, despite its historic potential to challenge and change the minds of Americans. “

The case has been brought by two gay couples, Kris Perry and Sandy Stier, of northern California, and Paul Katami and Jeff Zarrillo, who live in southern California.

Their lawyers, Ted Olson and David Boies, are expected to argue that banning gay marriage is not a legitimate government interest and that civil unions are not an adequate substitute.

Gay marriage was legalised in California in May 2008. Voters banned it under Proposition 8 the following November.