A 19-year-old man is fighting for life after suffering burns to his head, torso, arms and hands in a fire at a single-storey automotive workshop in Perth.

St John Ambulance said the man was rushed to Fiona Stanley Hospital as a priority one patient on Monday afternoon.

A hospital spokeswoman told AAP the man was in a critical condition.

A second man suffered superficial burns to his arm but was not required to go to hospital.

The fire started at Precision Automatic Transmissions on Ruse Street in Osborne Park about 12.30pm and was fully extinguished by 4pm.

The Department of Fire and Emergency Services said the cause of the blaze was accidental.

It caused heavy smoke and people were warned to avoid the area, but the plume cleared up after a few hours.