Minister for Crime Prevention James Brokenshire wrote to festival organisers asking them to warn people how such substances could be dangerous.
He also said drug laws would be changed so temporary bans could be introduced on "emerging substances" while scientific advice is sought.
Letters were also sent to councils and police forces in festival areas.
Mr Brokenshire said: "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'.
"That is why we are asking festival organisers and police to work with us to send out the message that these substances may not be safe and could contain illegal drugs.
"We are going to change our drug laws so we can respond quickly to emerging substances by introducing a temporary ban while we seek full scientific advice."
Some drugs previously sold as "legal highs" have now been banned, including mephedrone, now a Class B drug.
The Home Office said that the drug naphyrone is branded as NRG1 and sold as "plant food" or "bath salts", but may contain one or more illegal drugs.
The government said it will not permanently ban a substance without receiving full advice from the Advisory Council on the Misuse of Drugs.
In the letter sent to festival organisers, Mr Brokenshire said: "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."
The government's advice has been welcomed by Melvin Benn, managing director of Festival Republic, which stages the Latitude, Big Chill and Reading and Leeds Festivals.
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."
Organisers of this summer's music festivals have been asked by the government to warn about "legal highs".