I have a damp patch in a ceiling corner and runs down to the ground

I have a damp patch which recently appeared this winter. It is at ceiling height in the corner of two walls, both of which are external. The property is a purpose built (late 1960s) ground floor flat. The patch spreads downwards towards another smaller damp patch which appears at the skirting board level directly beneath the higher patch. Both patches are wet to the touch and if left untreated quickly develop mildew. I have been using a dehumidifier to dry the walls out which works, but it needs to be on almost constantly at the moment and requires emptying every couple of days. I have noticed some minor cracks in the brickwork on the outside wall at about the same height as the patch and presume this to be the problem. What is the best way to treat this?