Six men have gone on trial at Bristol crown court accused of grooming and sexually assaulting vulnerable teenage girls in the city.

They were allegedly part of a large group of men who abused girls over several years. All six men deny the charges against them, which involved “multiple complainants”.

The trial is expected to last 12 weeks but reporting restrictions mean full details, including the defendant’s names, cannot be revealed.

After a challenge to the restrictions by media organisations including the Guardian, Judge Macmillan, who is overseeing the trial, allowed some information about the case and the nature of the charges to be reported, acknowledging there was a public concern about “grooming gangs”.

She said: “There is a particular public interest at the present in information about serious sexual offending carried out by ‘grooming gangs’ and in issues of transparency about such matters. These issues are, quite properly, subjects of significant public comment and concern.”

Earlier this year, the prosecution applied for a postponement of all reporting of the trial until all verdicts had been returned. It argued that reporting risked prejudice to the administration of justice. The postponing order was granted.

This week a number of media organisations challenged the ruling, arguing that the trial should be reported as it happened in the interest of open justice.

But the judge said she did not believe it was possible to lift the restriction, expressing concerns that there was a significant risk witnesses’ evidence could be contaminated.

She said: “Given the level of public interest this trial is likely to attract, I am satisfied that any contemporaneous reporting of evidence in this trial is likely to come to the attention of witnesses and would result in a significant risk of witness contamination. This, in turn, would undermine the prospect of a fair trial.”

She added: “I am satisfied that revoking the existing order would give rise to a risk of serious prejudice to the administration of justice in this trial.”

The judge varied the existing order so that the fact of the trial, the general nature of the charges and the existence of her order could be reported.

The trial continues.