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This article is about the Statute of Westminster relating to the British Empire and its dominions. There were also Statutes of Westminster of 1275, 1285 and 1290 (known as 'First', 'Second' and 'Third'), relating to the government of the Kingdom of England.
The Statute of Westminster 1931 is an Act of the Parliament of the United Kingdom (22 & 23 Geo. V c. 4, December 11, 1931) which established a status of legislative equality between the self-governing dominions of the British Empire and the United Kingdom, with a few residual exceptions. The Statute became domestic law within each of the other Commonwealth Realms after the patriation of the particular Realm's constitution, to the extent that it was not rendered obsolete by that process.
The Statute is of historical importance because it marked the effective legislative independence of these countries, either immediately or upon ratification. The residual constitutional powers retained by the Westminster parliament have now largely been superseded by subsequent legislation. Its current relevance is that it sets the basis for the continuing relationship between the Commonwealth Realms and the Crown.

Parties
The Statute gave effect to certain political resolutions passed by the Imperial Conferences of 1926 and 1930, in particular the Balfour Declaration of 1926. One of the effects was removing the last imperial bond of power of British Parliament over dominions. The Colonial Laws Validity Act 1865 was repealed in its applications to the dominions. After the Statute was passed, the British government could no longer make ordinary law for the dominions, other than at the request and with the consent of that dominion.
It did not, however, immediately provide for any changes to the legislation establishing the constitutions of Australia, Canada and New Zealand. This meant, for example, that many constitutional changes continued to require the intervention of the British Parliament, although only at the request and with the consent of the Dominions as described above. These residual powers were finally removed by the Canada Act 1982, the Australia Act 1986, and the New Zealand Constitution Act 1986.
The key passage of the Statute provides that:
No Act of Parliament of the United Kingdom passed after the commencement of this Act shall extend or be deemed to extend, to a Dominion as part of the law of that Dominion, unless it is expressly declared in that Act that that Dominion has requested, and consented to, the enactment thereof.
It was also enacted that:
No law and no provision of any law made after the commencement of this Act by the Parliament of a Dominion shall be void or inoperative on the ground that it is repugnant to the law of England, or to the provisions of any existing or future Act of Parliament of the United Kingdom, or to any order, rule, or regulation made under any such Act, and the powers of the Parliament of a Dominion shall include the power to repeal or amend any such Act, order, rule or regulation in so far as the same is part of the law of the Dominion.
Under the provisions of section 9 of the statute, the British Parliament still had the power to pass legislation regarding the Australian states, although "in accordance with the [existing] constitutional practice". In practice, these powers were not exercised. For example, in a referendum held in Western Australia in April 1933, 68% of voters voted for the state to leave the Commonwealth of Australia with the aim of becoming a separate Dominion within the British Empire. The state government sent a delegation to Westminster to cause the result to be enacted, but the British Parliament refused to intervene on the grounds that it was a matter for the Commonwealth of Australia. As a result no action was taken. These residual powers were removed by the Australia Act 1986.