The discovery was apparently made by asbestos removal experts Kuston Vorland on Tuesday afternoon, with Wales Online saying the Cardiff Reptile Centre believe the suspected Chilean rose tarantula could now be twice the size of its shed skin.
The skin was originally found after experts moved into a darkened loft area of an asbestos-contaminated abandoned house in the Roath area of Cardiff.
Surveyor Katie Parsons-Young says she lifted up a floorboard in the 19th century building, only to discover a large hairy leg in the darkness.
She told Wales Online: “We had lighting in there so we moved the lighting to the other area of the attic where I was and could see there was something...I was the first in. I sort of saw a leg, screamed and went.”
The website reported that the majority of the team fled the building after the discovery, while a handful of braver souls stayed behind to pick up the skin - which they are said to have believed was a dead tarantula at the time.
It was only after the team took the skin to Cardiff Reptile Centre that they realised exactly what they had found - with experts there apparently saying it is likely the creature is still at large.
Tarantulas also swell in size after shedding, meaning it could now be up to twice as big as the discarded skin. The fact the skin was found in an asbestos-contaminated building means the spider could still be coated in the carcinogen.
It is not known if the tarantula was a lost pet or if there is a breeding population in the area.
Confirmation of whether the skin was indeed infected with asbestos is expected when results come back from a laboratory later this week, but if it is found to contain traces of cancerous mesothelioma, Wales Online say it likely to be a world-first.