The fire wiped out wildlife and shrubbery in an area equivalent to 28 football pitches as dozens of firefighters tackled the blaze.
Michael Payne, a forest ranger, revealed the cause of the fire: "A man decided to cook his lunch with a camping stove on a bridal path and a blade of grass caught light and blew off into the gorse."
The fire on Sunday 7 June shrunk the size of the park - which is in Ashdown Forest, East Sussex, and is known outside A. A. Milne's cherished books as the Five Hundred Acre Wood - by around 16 acres.
The forest ranger added: "The ignition [on the camping stove] was nigh on simultaneous and he couldn't contain it. The flames were up to 30ft tall in places.
"It was all burning through dense gorse which made tackling the fire very difficult.
"Don't use any sort of naked flame out in the forest. Not only could you be prosecuted for breaking by-laws but you could cause significant damage to the forest."
The ranger warned that wildlife, reptiles, insects, birds and butterflies would have died in the blaze.
The 16-acre area could take up to ten years to grow back, he added.
Forest by-laws do not permit the use of open flames, but the ranger could not confirm if the culprit would face a fine.
The devastation could have been far greater if it was not for fire crews from four stations who tackled the blaze for around eight hours.
The blaze took place two years to the day after another fire ripped through the country park destroying 20 acres of grassland.