Two revellers who died and another who is in a critical condition were among 15 people to attend hospital during one night of the Mutiny Festival.
An 18-year-old woman and a 20-year-old man fell ill on Saturday before organisers of the festival in Portsmouth issued a warning of a "high strength or bad batch" of drugs.
The second day of the festival - which Craig David and Sean Paul were to headline - was cancelled as a "safety precaution".
It is with tremendous sadness that we can confirm that two of our festival family died in hospital overnight.An 18...
A spokeswoman for Queen Alexandra Hospital, where the pair died, said it is believed they were admitted after falling ill from the effects of drugs. Another person remains in a critical condition.
A further 12 concertgoers attended the hospital on Saturday after being at the festival, but they were "not necessarily all drugs-related", she said.
Three more attended on Sunday morning to be treated from the injuries sustained in assaults or because of alcohol-related intoxication, the spokeswoman added.
The deaths are being treated as separate incidents and are not being treated as suspicious but Hampshire Constabulary are investigating.
We can confirm that two people have died after falling ill at Mutiny Festival in Portsmouth.An 18-year-old woman and...
Mutiny warned at around 9.30pm on Saturday, after the pair were taken ill, that it was "aware of a dangerous high-strength or bad-batch substance on site", and that no drugs should be taken.
A subsequent statement from the festival said: "Mutiny Festivals are devastated to hear of the tragic loss of life from its festival family today."
It later added: "Following the terrible news from earlier today, the team behind Mutiny Festival are incredibly sad to announce that the Sunday of the festival has been cancelled as a safety precaution."
The organisers of the festival, which was headlined by Dizzee Rascal on Saturday, also urged customers to "responsibly dispose" of any substances.
Police said the woman was taken ill at the festival in King George V Playing Fields at 7.10pm before the man was found collapsed 20 minutes later.
"The deaths are being treated as separate incidents at this stage. They are not being treated as suspicious but inquiries are being made to determine the circumstances of what happened in each case," a force spokesman said.
The force said next-of-kin for both the woman and the man had been informed and were being supported by specialist officers.
Anyone with information is asked to contact police on 101.