A government minister has asked festival promoters to warn ticket-holders about the potential danger of legal highs, the Guardian newspaper has reported.
In a letter to organisers, James Brokenshire wrote that some substances that are sold as safe could be hazardous and illegal.
Brokenshire wrote: "As part of your preparations for this year's event, I ask you to review the measures you have put in place to ensure that your festival is as safe an environment as possible, to help protect the public, especially young people, from the potential harms and risks of all drugs."
"During the festival season, we know that people may be tempted to try potentially dangerous new drugs, particularly when they are advertised as 'legal' or 'herbal'.
"We are asking festival organisers and police to send out the message that these substances may not be safe and could contain illegal drugs."
Festival Republic man, Melvin Benn, who helps run Reading, Leeds and Latitude festivals has taken the advice onboard.
He said: "I can confirm that we do not allow legal high traders to trade at any Festival Republic festival and fully support the minister's view."