On 31 May 1910 these four colonies came together to form the Union of South Africa and the individual colonial flags were no longer used and new South African flags came into being. Once again, as a British dominion the British Union Flag was to continue as the national flag and the standard British ensign pattern was used as a basis for distinctive South African flags.
As was the case throughout the British Empire, the Red and Blue Ensigns were the official flags for merchant and government vessels at sea, and the British Admiralty authorised them to be defaced in the fly with the shield from the South African coat of arms. These ensigns were not intended to be used as the Union's national flag, although they were used by some people as such. Although these ensigns were primarily intended for maritime use, they were also flown on land.
The Blue Ensign was flown over the Union’s offices abroad between 1910 and 1928.