The festival attracts thousands against a backdrop of Snowdonia

Meic Stevens, the Saw Doctors and Meinir Gwilym are performing at the Tan y Ddraig concert on Monday 29 August.

Organisers said people will have to be over 21 to bring alcohol onto the site, or to buy drinks from the bar.

The festival, which was started by singer Bryn Terfel and starts next Friday, in is its sixth year.

The ban was agreed by festival organisers and North Wales Police, after problems at previous concerts.

We're trying not to encourage underage drinking on the night and we also don't want to spoil the fun of the people who are coming along to enjoy the music Organiser Steve Porter

Chief Inspector Derrick Roberts said that, if the situation did not improve, a total alcohol ban could be brought in. "If things don't improve this year there will be no alcohol next year," he said.

"Over the past two years at this event we have found that young persons have been highly intoxicated.

"Children as young as 14 have been found to be under the influence of alcohol in the past, although it has been identified that the alcohol has not necessarily been bought at the bar but brought into the field.

"We clearly needed to take action."

Organisers said they were faced either with a complete ban on alcohol or this rule about over-21s.

Steve Porter, the event's executive producer, said the most important thing was ensuring the safety of the audience.

Van Morrison will perform at the concert

"It's something we've considered looking at over the five years we've managed the festival.

"It's something that if it's not dealt with, it could become an issue.

"It's an evening when all sorts of people of all ages come - we're trying not to encourage underage drinking on the night and we also don't want to spoil the fun of the people who are coming along to enjoy the music."

Van Morrison, Bryn Terfel, Katherine Jenkins, Aled Jones and Andrea Bocelli are all due to perform at the four-night festival.