What was the first soap (washing) powder?

From the research I have done the answer appears to be Tide. Although I seem to think that Rinso may have been marketed first.

http://inventors.about.com/library/inventors/blsoap.htm

I will say Tide for the following reasons.

Proctor and Gamble in 1933, this discovery was introduced in a detergent called "Dreft," but it could only handle lightly soiled jobs. The next goal was to create a detergent that could clean heavily soiled clothes. That detergent was Tide ... October 1946 as the world’s first heavy-duty detergent.

http://inventors.about.com/library/inventors/blsoap.htm

The breakthroughs in the development of household detergents come in 1946: the first built detergent (containing a surfactant phosphate builder combination) was introduced in the United States. Phosphate builders vastly improved detergent performance, making them suitable for heavy duty cleaning. This was Tide.

ihmservices.com/prodhistory.htm no longer exists

Those are mostly detergents rather than soap powders. There is quite a difference. According to https://en.wikipedia.org/wiki/Robert_Spear_Hudson_(soap) the answer is Hudson's Dry Soap, which started in Hudson's shop in West Bromwich (founded 1837). The business later moved to Liverpool, and then became part of Lever Brothers. Rinso is a soap powder, a later brand from Hudsons/Levers, mostly replaced by the detergent Surf.

Soap powders are powdered (or flaked) soap. Most of the cleaning powder things on the market are detergents, which contain any permutation of many things - except soap. https://en.wikipedia.org/wiki/Detergent_powder Tide is from 1946, and is a detergent - and is ruled out on two counts as the answer here. https://en.wikipedia.org/wiki/Procter_%26_Gamble