North Yorkshire Fire Service said the blaze had shrunk overnight, but by 12.30pm it had spread to cover 5sq km and was declared a major incident. Pictures posted on Facebook by Bedale fire station showed the fire burning overnight, while drone footage filmed on Tuesday showed huge flames and vast clouds of smoke.
More resources are now being sent to the scene in an effort to bring it under control.
The hot, dry weather in the UK has made such fires more likely and a fire on heathland in Dorset was also declared a major incident at the weekend.
Crews were still on site at Holt Heath today, dealing with hotspots and occasional flare-ups sparked by wind.
However, Dorset and Wiltshire Fire Service said "good progress" was being made across all four sectors of the incident.
European nations have been dealing with much more destructive fires this summer in what is becoming a growing problem — especially in the south of the continent.
At least 1,400 firefighters were deployed in France's southern Aude region last week to try to stop the country's biggest wildfire in decades from reigniting. About 7,700 residents were also evacuated from near the Greek city of Patras, west of Athens.
People were also ordered to move as fires spread on the popular holiday islands of Chios and Kefalonia.
