Three people suffered "serious side-effects" after taking a tablet they believed to be a class A drug at the V Festival in Weston Park, Staffordshire.
Staffordshire Police said: "Emergency teams have treated people this evening who took a blue tablet they believed to be ecstasy at V Festival in Weston Park. All had serious side-effects after taking the tablets and needed urgent medical treatment."
The tablets were known as "Blue Ghost" and police said they had already seized a small number of them.
It is believed the trio reported stomach problems, though none required hospital treatment.
A festival spokesman said: "We take these reports seriously and will do everything in our power to support the police in their investigations to ensure the festival continues to be a safe environment for everyone."
The organisers of a music festival where a woman died have expressed their condolences to her family.
BoomTown fair would like to express their sincerest condolences to the family of the young woman who was found unconscious on site but despite the efforts of all our staff involved she was later pronounced dead shortly after arriving at Winchester hospital.
Our thoughts are with all her loved ones at this tragic time and the festival is doing all it can to assist those involved.
– BoomTown Fair
Police said her death at Matterley Estate, near Winchester, yesterday is not believed to be suspicious.
A woman has died at the BoomTown fair at Matterley Estate, near Winchester, according to local media reports. The 31-year-old was found unconscious in a toilet block just after lunchtime yesterday. Her death is not believed to be suspicious.
Local media reports quote a police spokesperson confirming the death yesterday. It is the second year in a row that a woman has died at the festival. Last year Ellie Rowe, 18, from Glastonbury in Somerset, died after taking the drug ketamine.
Musical festival bosses are being urged to ban 'hippy crack' from their events this summer because it can cause death.
The laughing gas, which is sold in balloons for around £2 per dose, is inhaled to make people feel euphoric and relaxed. But drug experts warn it can cause death on first time use.
Norman Baker, a Liberal Democrat MP, wrote in a letter to festival organisers: "Nitrous oxide can slow down both brain and body response and there is even a risk of death from a lack of oxygen especially if the substance is consumed in a small space or a substantial amount is rapidly used."
He added: "If you have not already done so, I also encourage you to consider a ban on the sale of this substance and where possible take measures to prevent it from being used within the festival site."
The headliners playing at Reading and Leeds Festival this year have finally been announced. The Cure, Kasabian and Foo Fighters will be gracing the stage in August, alongside Florence and the Machine and The Vaccines.