The Statute of Westminster 1931[a] is an act of the Parliament of the United Kingdom that sets the basis for the relationship between the Dominions (now called Commonwealth realms) and the Crown.

Its modified versions are now domestic law in Australia and Canada; it has been repealed in New Zealand and implicitly in former Dominions that are no longer Commonwealth realms.

Oversight of these colonies oscillated between relatively lax enforcement of laws and centralisation of power depending on the politics of the day, but the Parliament in Westminster always remained supreme.

Australia was similarly deemed a dominion when it federated in 1901, as were Newfoundland, New Zealand, South Africa, and the Irish Free State in the first decades of the 20th century.

King George V expressed his desire that the laws of royal succession be exempt from the statute's provisions, but it was determined that this would be contrary to the principles of equality set out in the Balfour Declaration.

Since 1931, over a dozen new Commonwealth realms have been created, all of which now hold the same powers as the United Kingdom, Canada, Australia, and New Zealand over matters of change to the monarchy, though the Statute of Westminster is not part of their laws.

That authority remained in effect until the Constitution Act, 1982, which transferred it to Canada, the final step to achieving full sovereignty.

[14] As a consequence of the statute's adoption, the Parliament of Canada gained the ability to abolish appeals to the Judicial Committee of the Privy Council.

[16] The passage of the Statute of Westminster meant that changes in British legislation governing the succession to the throne no longer automatically applied to Canada.

[19] In 1924, Kevin O'Higgins, the Free State's Vice-President of the Executive Council, declared that "Ireland secured by that 'surrender' [the Treaty] a constitutional status equal to that of Canada.

[23] Originally, the UK government had wanted to exclude from the Statute of Westminster the legislation underpinning the 1921 treaty, from which the Free State's constitution had emerged.

The other Dominions backed Cosgrave and, when an amendment to similar effect was proposed at Westminster by John Gretton, parliament duly voted it down.

"[25] He went on to present the statute as largely the fruit of the Free State's efforts to secure for the other Dominions the same benefits it already enjoyed under the treaty.

Upon taking office, de Valera began removing the monarchical elements of the Constitution, beginning with the Oath of Allegiance.

De Valera initially considered invoking the Statute of Westminster in making these changes, but John J. Hearne advised him not to.

Robert Lyon Moore, a Southern Unionist from County Donegal, challenged the legality of the abolition in the Irish Free State's courts and then appealed to the Judicial Committee of the Privy Council (JCPC) in London.

The remaining role of the British Parliament was removed by the New Zealand Constitution Act 1986 and the Statute of Westminster was repealed in its entirety.

The second paragraph of the preamble to the statute reads: And whereas it is meet and proper to set out by way of preamble to this act that, inasmuch as the Crown is the symbol of the free association of the members of the British Commonwealth of Nations, and as they are united by a common allegiance to the Crown, it would be in accord with the established constitutional position of all the members of the Commonwealth in relation to one another that any alteration in the law touching the succession to the throne or the royal style and titles shall hereafter require the assent as well of the parliaments of all the Dominions as of the Parliament of the United Kingdom:Though a preamble is not considered to have the force of statute law, that of the Statute of Westminster has come to be a constitutional convention, which "has always been treated in practice as though it were a binding requirement".

The King wanted to marry Wallis Simpson, whom Baldwin and other British politicians considered unacceptable as Queen, as she was an American divorcée.

Baldwin was able to get the then-five Dominion prime ministers to agree with this and, thus, register their official disapproval at the King's planned marriage.

The King later requested the Commonwealth prime ministers be consulted on a compromise plan, in which he would wed Simpson under a morganatic marriage, pursuant to which she would not become queen.