The Statute of Westminster, 1931 is an Act of the Parliament of the United Kingdom and separate versions of it are now domestic law within Australia and Canada; it has been repealed in New Zealand and implicitly by subsequent laws in former Commonwealth realms. Passed on 11 December 1931, the act, either immediately or upon ratification, effectively both established the legislative independence of the self-governing Dominions of the British Empire from the United Kingdom and bound them all to seek each other's approval for changes to monarchical titles and the common line of succession. It thus became a statutory embodiment of the principles of equality and common allegiance to the Crown set out in the Balfour Declaration of 1926. It thus had the effect of making the Dominions sovereign nations.

Source: WIKIPEDIA