Flag of the Western Union from 1949 until the organisation was transformed into the Western European Union in 1954.
Flags of the World:

"The flag has a dark blue field bearing five links forming an unbroken chain, in gold (the links form a kind of pentagon, but with the "point" bottommost). The field has a multi-colored border: red (outside), gold, black, and white [The relative proportions of the border are approximatively: Red 3, each of the others 1. The total width of the border is approximatively half that of the depth of the flag.] [...] These colors were taken from the national flags of the member countries, namely, Great Britain, France, Belgium, Holland [sic] and Luxembourg."
Own work based on: description on [1]
Ssolbergj