Dozens of firefighters have been tackling the blaze, which started on the edge of the moor in Greater Manchester, in temperatures of about 27C (80F) this week.
Police have now confirmed they are treating the fire as arson, saying they have received witness reports of people lighting a bonfire on the land near Stalybridge on Sunday June 24.
“The moorland fire is now being investigated as arson following information from local witnesses and initial inquiries,” said Chief Superintendent Neil Evans, territorial policing commander for Tameside.
“The scale of this fire is exceptional and we are treating it with the utmost seriousness.
“That said, we are under no illusion that solid evidence as to where the fire started and what was the cause will not be easy to establish.”
Firefighters are still at the scene.
“Although now under control, the fire is continuing to burn despite the best efforts of all those involved,” a police statement said.
Meanwhile, Greater Manchester firefighters and others from brigades including Tyne and Wear, the South West, Nottinghamshire, Humberside and Warwickshire are continuing to support colleagues from Lancashire who are tackling a similar blaze at Winter Hill, near Bolton.
Arsonists were spotted lighting new fires in the area as the fire burned, Sky News was told this week.
A 22-year-old man arrested on suspicion of starting a fire has been released under investigation.