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The CCTV footage shows the hooded arsonist emptying at least one bottle of flammable liquid into two rooms near the gym's entrance.

The criminal then lights up an unidentified object and tosses it into one of the rooms.

The scale of the explosion appears to have taken the fire bug by surprise with the room exploding in fire.

Some of the flames even reach the arsonist who races for the door, leaving his bag behind.

Police said the suspect may have been injured.