The blaze broke out in the West Car Park at the Winchester festival at around 4.15pm and took firefighters some two hours to bring under control control.
A think black plume of smoke rose into the air over the festival site on Friday afternoon, which could be seen from Winchester itself.
On Monday, Hampshire Fire and Rescue Service (HFRS) said the fire was likely caused by a discarded cigarette which set light to stubble around the parked cars.
The fire left dozens of cars burned out. No-one was injured in the incident.
Six pumps and two water carriers from around Hampshire were called to the blaze, including the South Central Ambulance Hazardous Area Response Team and Hampshire Constabulary.
A campsite adjacent to the car park was evacuated as a precaution but was reopened to the public shortly after the blaze.
BoomTown's event management team opened an incident information point to assist those affected by the fire and provided phones and internet access so they could arrange alternative transport home.
They tweeted to say the vehicles affected arrived on Thursday (August 11) between 10am and 11am.
Hampshire Constabulary said it was working with HFRS to investigate the cause of the fire.