The gable end of an old cotton mill just off Victoria Street in Nelson has subsided into the Leeds and Liverpool Canal.
David Walker, who was walking his dog along the canal towpath when he noticed the damage, said: “It must have happened between Thursday last week and Monday morning.
“It had fallen into disrepair in the last few years. I was shocked, quite frankly, to see the extent of the damage so close to a bridge that is much used by the public.
“A lot of the buildings on the canalside are an eyesore due to lack of maintenance. It might even be a problem for canal users because I’m not sure how much has gone into the water.”