The ground level being higher on the outside of the wall, this allows the moisture from the soil on the opposite side of the wall to penetrate through the wall causing damage to the internal wall.
A common cause of penetrating damp is the incorrect installation, deterioration or omission of the vertical damp course.
Evidence of penetrating lateral damp:
Paint flaking and bubbling, and ending suddenly on a horizontal level. Discolouration of paint.
Efflorescent salting present on the surface.
Strip the plaster back to the brickwork on the walls concerned.
Apply waterproofing plaster/ chemicals to the brickwork and allow to dry.
If applicable this process needs to be repeated.
Prime and paint the area worked on.