Part of disused Putney Hospital has collapsed forcing Wandsworth Council to bring in demolition experts.
Concerned dog-walkers and residents inundated the Wandsworth Guardian news desk on Monday morning after spotting the building's crumbling walls tumbling to the floor.
The site, situated on the north side of Putney Common, is enclosed by high walls but the noise of the collapse drew a number of worried bystanders to the scene.
A spokesman for the council said: "An internal wall, within the vacant nurses' accommodation block at the rear of this long-derelict hospital site, has collapsed.
"In order to make the rest of the building safe our dangerous structures contractor is now carrying out limited demolition work."
The hospital closed services to patients in 1998 and has remained vacant ever since.