We had this problem 50 years ago in our council house. Council called in the experts, a private company, and they told me what newboy has said, that to stop efflorescence, they must first find the water source because it is water borne. They did 3 things:
1 They renewed the DPC.
2 They filled the wall cavity with foam.
3 They sprayed the wall with some sort of colourless sealant.
Although this seemed to cure the problem, the question remained as to why it started in the first place because the house wasn't a new build. The cause turned out to be an underground stream had been blocked somehow further down its course.