It was originally proposed and adopted by Marconi on January 7, 1904. It was officially superseded with the code SOS in 1908 which was considered more distinctive and easier to use.
The signal used by British radio operators for many years was CQD (based on the attention call CQ), but there was no international standard. At the 1906 International Conference on Wireless Communication at Sea, it was resolved that SOS should be used as a distress call. Britain adopted this standard in 1908, but the radio operators retained their old habit of using CQD. When the Titanic sank in 1912, its radio operator Jack Phillips initially sent the distress call as "CQD", but was reminded by Harold Bride, the junior radio operator that the new code was "SOS" and that he should send it, as it might be his "last chance to use it." Phillips then used both codes alternately. For some reason, people are under a mistaken belief that the sinking of the Titanic was the first use of the "SOS" call: it wasn't. However the news accounts of the Titanic disaster cemented the new "SOS" call in the mind of the public, and it began to be used regularly afterward.