Sunlight striking a shaving mirror triggered a blaze at the home of Dame Joan Collins.
The concentrated rays seared into a blind and led to a fire in the actress's London flat.
Firefighters rushed to the property in Belgravia after intense sunlight sparked flames in the en-suite bathroom.
The 85-year-old Dynasty star and her husband, Percy Gibson, fled their home after trying to put out the blaze with a fire extinguisher, and escaped serious injury.
London Fire Brigade (LFB) tackled the flames on Saturday afternoon after being called to the scene.
Dame Joan tweeted her thanks to the emergency services for dealing with the fire and providing treatment for smoke inhalation.
Firefighters have warned other people to beware of the intense sun to avoid suffering a similar incident.
Crew manager Chucks Abili, who was at the scene, said: "These sort of fires are not as rare as you would think and this incident shows that fires can happen in anyone's home.
"Particularly at this time of year, the sun is low in the sky so the concentrated rays can set light to nearby items. We've seen fires caused by glass doorknobs, jars and bottles on windowsills, and hanging crystals.
"Our advice is to always keep crystals, glass objects and mirrors away from windowsills and out of direct sunlight.
"This is also a reminder of the importance of having smoke alarms fitted throughout your home, as they give the earliest possible warning of fire.
"Although Mr Gibson did a good job in tackling the fire initially and managed to stop it spreading, he could have been more seriously hurt. We would always advise people to leave their property immediately and dial 999."
Dame Joan wrote on Twitter: "THANK U from the bottom of my to the marvelous @LondonFire who attended to the terrifying fire yesterday in our flat. #threealarmfire.
"My hero Percy had already doused the flames consuming the entire wall with handheld extinguisher (#donttakethisrisk). #gratitude #firefighters."
A second message said: "And thanks also to the @NHSEnglandLDN #nhs #emt #Ambulance crew who took care of our smoke inhalation and comforted me.
"And the wonderful @metpoliceuk who kept the street closed and everyone safe. #gratitude."
LFB has advised that people keep mirrors and crystal or glass objects away from sunny windowsills, fit at least one frequently-tested smoke alarm per floor, and close the door on any fire, leave the property, and call 999.