Park bosses found an abandoned barbecue grate near the site of the fire on the National Trust for Scotland's (NTS) Mar Lodge Estate, near Braemar, in Aberdeenshire, not far from Balmoral Castle.
The blaze broke out yesterday afternoon and centred on Glen Lui in the 72,000-hectare estate.
At its height, the blaze was fought by 11 fire crews using 10 appliances drawn from across the north of Scotland, aided by 14 NTS staff and 15 workers from surrounding privately-owned estates.
A helicopter was used to water bomb the blaze, which was brought under control by 7pm.
The fire destroyed blaeberry and scrub undergrowth among tracts of mature Caledonian pinewoods, however NTS said the damage to mature trees appears to be limited as the fire did not reach canopy level.
Park bosses urged people not to use open flames in the countryside, particularly during hot weather.
David Frew, NTS property manager for the Mar Lodge Estate, said: "It is frustrating to report that there is a very high likelihood the fire was caused by carelessness. The cause of ignition appears to be a fire at a campsite where we found an abandoned barbecue grate.
"This is a completely needless situation and a reminder that anyone embarking on a trip into the countryside, particularly during a dry warm spell, must take care and avoid the use of open flames.
"It seems we have been relatively fortunate as things could have been a lot worse. Some patches are still smouldering, and ongoing dry and breezy conditions for the next few days might lead to re-ignition and we are remaining vigilant for that reason.
"The best thing visitors can do in the meantime is to use their common sense and follow well publicised good practice if they plan to camp out in the area."
Mar Lodge Estate recently featured in the BBC's Winterwatch TV programme and is set in the Cairngorms National Park.
It covers some of the most remote and scenic wild land in Scotland, including four of the five highest mountains in the UK.
Will Boyd Wallis, Cairngorms National Park Authority's head of land management and conservation, said: "The Cairngorms National Park is an extremely important place for nature and wildlife so the message from us is really quite simple - never light a fire during long dry spells of weather or in woodland or on peaty soil.
"We want people to enjoy the Cairngorms National Park and have a great holiday, but not at the expense of our wildlife and the important habitats that support them."