Damp wall - Pigletjohn?

[Dampwall]

We have just moved into a new house and this has now appeared on the wall behind the fridge freezer - we noticed it because it has spread along the wall.
Behind the damp wall is the utility room where the washing machine lives but that side is perfectly dry.
Thoughts? I assume we are going to have to hack off the plaster and investigate? Get a plumber in? @Pigletjohn?
And yes, the plug looks dodgy too 🙄

To be fair what would a plumber do if there is not a leak. Most damp is caused by condensation. water condensating at a cold point when there are high moisture levels.how old is the house?

It's a Victorian house. This is an internal kitchen wall.
Would the fridge cause if? The previous owners had the fridge here.

It's a bit hard to tell. Being a Victorian house it won't have a DPC but don't try to retrofit one it's always a disaster. It will be gypsum plaster when it should be breathable lime, Behind the fridge should be warm but perhaps there is moisture from a leaking appliance there. Tbf in an old property (and I live in an older one) I would just live with it unless it was causing mould. Not much help sorry

the stain is yellow, suggesting a leak, and an attempt has been made to hide it by replastering and roughly painting. It is not a condensation patch.

If the damp patch is semi-circular (which it is, roughly) then the source of water is probably about the centre of the circle. As the washing machine is on the other side, it might be a pipe in the floor or wall. Is it a concrete floor?

Take up any floor covering in both rooms and see if the floor is wet. Puzzle out the routes of the various pipes. Please post more photos on both sides of the wall, including the pipes and any nearby drains

I think the wooden floor in your pic is already damaged. Is it laminate or old parquet? You can lift parquet, one block at a time. It might be stuck down with bitumen.

pencil a straight level line 100mm above the floor. You can chip off all the plaster below this line, because when you fit new skirting, it will be hidden. This will expose the brickwork, and you may see more clearly what is happening. Exposing the brick will also allow water to evaporate from the surface.

It is not "rising damp" if such a thing exists because it is not all along the wall, but it is a damp spot with a centre.

I think it's probably a pipe leak.

btw old lime plaster is usually creamy white, may have gritty sand in it except on the smooth surface. Modern gypsum plaster is usually pink if dry, brown if wet. Sometimes it is grey.