Ashdown Forest - the inspiration for Winnie the Pooh's Hundred Acre Wood - went up in flames overnight as a fire swept across six acres of land.
Firefighters were called to the forest near Kingstanding, 19 miles north-east of Brighton in East Sussex, at about 9.30pm.
Crews are still battling the blaze, which has now spread across six hectares, an area roughly equivalent to six full-size football pitches.
The forest gave birth to AA Milne's famous creation. The Hundred Acre Wood, from the author's children's books, is actually named after the Five Hundred Acre Wood in Ashdown Forest.
An East Sussex Fire and Rescue Service spokesman said: "Six fire engines, four Land Rovers, a water carrier and the control unit are in attendance and our teams are tackling the fire using beaters."
The cause is not yet known, but like some wildfires already reported this year, it is feared people lighting disposable barbecues in the forest or tossing cigarette ends into the undergrowth sparked it.
But firefighters have not yet ruled out arson.
Nature lovers fear the inferno has destroyed many birds' nests containing eggs and chicks, and has killed rare butterflies and other wildlife.
Pictures shared on social media show towering flames tearing across the High Weald Area of Outstanding Natural Beauty.
The fire is the second in as many months to ravage the forest.
On February 26, during the heatwave, two separate blazes started within an hour of each other in the forest on the hottest February day on record.
Firefighters said more than 35 hectares had been ravaged.