Three people died after attending events over the bank holiday weekend, including two teenagers from suspected drug overdoses.
The death of a man who fell ill at Reading Festival is being treated as unexplained.
Emergency services were called to an area near one of the Berkshire music festival’s stages just before 11pm on Saturday, Thames Valley Police said.
The man was taken to Royal Berkshire Hospital at about 11.40pm, but he died the next day.
Reading Festival’s police commander Chief Superintendent Rob France said: “We are investigating the circumstances of how he became ill and his death is being treated as unexplained at this time.
“We will continue to provide support to the man’s family at this difficult time.”
Meanwhile, at Leeds Festival - which takes place on the same dates as Reading Festival and features the same acts - a teenage boy was arrested after a 17-year-old girl died of a suspected drug overdose.
The suspect, also 17, was detained on suspicion of supplying controlled drugs.
Both teenagers are from the Oldham area, West Yorkshire Police said.
Officers were called to the Bramham Park site following a report that the girl had died shortly after 3.40am on Saturday.
Leeds Festival’s police commander Superintendent Matt Davison said: “While the exact cause of her death is yet to be established, the information we have indicates that she has taken a combination of drugs.
“This is a tragic incident and I hope serves as a timely reminder to others of the risk of taking illegal substances, particularly when mixing different types of drugs and drinking alcohol.”
Melvin Benn, from Festival Republic, which runs the event, said: “To say we are deeply saddened about this tragedy is an understatement.”
A day earlier, the festival had posted a warning about “dangerous, high-strength pills” in circulation at the site.
They wrote: “Donkey Kong, Trump and Skype pills have been found in circulation on site. These are very dangerous, high-strength pills, up to three times the normal average adult dose.”
On the festival’s website, organisers state that the event “does not condone the use of drugs”, adding: “It is illegal to buy, sell or take drugs. Drugs enforcement laws are as applicable on site as anywhere else in the UK.”
Elsewhere in Leeds, another teenager died of a suspected drugs overdose after collapsing at a nightclub.
West Yorkshire Police said officers were contacted at 3.18am on Monday by staff at Leeds General Infirmary who said an 18-year-old woman had been taken in by ambulance after collapsing at The Warehouse nightclub.
A force spokesman said the woman was pronounced dead a short time later.
He said a 19-year-old man was arrested on suspicion of being concerned in the supply of Class A drugs.