The blaze spread across properties in Middlesex Road, Southsea, on Monday evening.
Sunlight reflecting off a glass panel onto one of the sheds, causing it to ignite.
Tor Strowger, Station Manager at Southsea Fire Station, said: ‘Although these type of incidents are fairly unique, over the past few weeks we have seen a number of fires caused by sunlight reflecting through glass.
‘Please take some time to have a look around your homes and gardens and ensure that any mirrors and glassware are kept out of direct sunlight or where the sun can reflect from them on to other items.’
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Firefighters used six breathing apparatus and a hose reel jet to quell the flames, and ventilation fans to clear the smoke.
Crew manager Simon Haswell said: ‘There was fairly significant damage caused to three sheds that were destroyed.’