Police have issued a warning to recreational drug users following the sudden death of an 18-year-old man at a party.
The teenager is thought to have inhaled an unknown illegal substance at the gathering on Chobham Common in Surrey.
Surrey Police urged other party-goers who may have taken the drug to go to hospital immediately if they display any worrying symptoms.
The force said it was informed of the death of the man, who was from the Farnham area, by the South East Coast Ambulance Service.
Chief Inspector Alison Barlow said: "Whilst a post-mortem examination still needs to be carried out to establish the cause of this boy's death, we are extremely concerned that it could be linked to him taking this drug.
"The advice to anyone who was at the event on Chobham Common who might have taken this drug and is exhibiting worrying symptoms is to attend A&E immediately.
"Similarly if you believe you may have taken this same substance elsewhere and are feeling unwell then please seek medical assistance.
"We also need the public's help in getting this potentially fatal drug off the streets of Surrey."
Anyone with information about the substance or anything that may help the ongoing investigation is asked to call Surrey Police on 101 or the charity Crimestoppers, anonymously, on 0800 555 111.
In a separate incident, two teenagers are in a serious condition in hospital after being taken to hospital with four others suffering from the effects of drug use in Glastonbury, Avon and Somerset Police said.