A careless smoker caused a fire that left a Calgary condo complex unlivable, fire investigators have concluded.

The massive blaze affecting 159 units at 23 Millrise Dr. S.W. on Thursday was sparked by a piece of smouldering material left in a balcony planter, the Calgary fire department said Monday.

It was the second time in a week that significant fire damage was caused by the careless disposal of smoking materials, officials said.

Careless disposal of smoking materials caused the fire at a condo complex in southwest Calgary on March 18, investigators say. (Submitted by Michelle Brown)

The Millrise fire was accidental, officials added. No estimate was released on the value of the damage.

On March 11, a cigarette tossed onto a pile of compost and debris sparked a fire at 171 Covewood Close N.E. that gutted one house and damaged two others. Damage was estimated in the hundreds of thousands of dollars.

Fire officials urged Calgarians to use common sense when disposing of cigarettes.

Cigarettes should be extinguished and properly discarded into a metal or glass container, officials said.