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Pictures taken near to the blaze show large plumes of smoke rising above the trees.
Strensall Common is a nature reserve about six miles (10km) from York city centre and is the only place in England where the rare dark-bordered beauty moth can be found.
Luke Charters, MP for York Outer, said: "Strensall Common is a place I care about deeply.
"Like many local residents, I treasure it for its natural beauty and peace."
He continued: "I'm deeply saddened by this incident and I hope the fire can be brought under control as soon as possible."
The MP paid tribute to the "outstanding" firefighters who responded with "courage, professionalism and dedication".
