A building at a Christchurch secondary school has been severely damaged in a suspicious fire early today.

Southern Fire shift manager Rewi Grace said the fire broke out in a small building at the southern end of Christchurch Boys' High School at 1.30am.

Firefighters arrived to find the property well ablaze and took two hours to extinguish the blaze.

Mr Grace said they were unable to get inside the burned building because of safety concerns arising from previous earthquake damage.

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Police and fire investigators were returning to the school today to investigate the cause of the fire.