The Parliament of the United Kingdom is dissolved 25 working days before a polling day as determined by the Fixed-term Parliaments Act 2011. This period was formerly 17 days, and the new 25-day period, operative for the first time in the General Election of 2015 (7 May), represents the longest United Kingdom election period without a parliament since 1924. Importantly, the Act does not affect the sovereign's power to prorogue Parliament, under the "Supplementary provisions" of the Act.
Members of Parliament cease to be so, as soon as it is dissolved, and they may not enter the Palace of Westminster, although they and their staff continue to be paid until polling day. Parliament is usually prorogued or adjourned before it is dissolved. Parliament may continue to sit for a wash-up period of a few days after the prime minister has announced the date when Parliament will be dissolved, to finish some last items of parliamentary business.
A royal proclamation is made summoning a new parliament, fixing the date when the new parliament is to assemble, and requiring the despatch of writs of summons to the Lords Spiritual and Temporal. Under the Fixed-term Parliaments Act, writs of election are now despached to the returning officers of each constituency automatically, by virtue of a provision contained in the statute (in concert with the fact that the royal proclamation no longer summons the holding of the election, but only the meeting of the new parliament). A general election must be held 17 days (excluding weekends and bank holidays) after the proclamation summoning Parliament is issued. By tradition, a copy of the royal proclamation is delivered by hand from the Privy Council Office to Mansion House in the City of London. It is then read out by the Common Cryer (also called Mace-bearer or Serjeant-at-Arms) of the City on the steps of the Royal Exchange in the heart of the City, having been handed to him by the Common Serjeant of the City, ahead of its being also read out in the London boroughs. This tradition was again carried out at the most recent dissolution, in May 2017.
The last dissolution of Parliament was on 3 May 2017, to make way for the general election to be held on 8 June 2017. It dissolved after a two-thirds majority vote by the House of Commons, as required by the Fixed-term Parliaments Act.
30 March 2015 was the first time Parliament had ever dissolved automatically, as opposed to being dissolved by royal proclamation (although provision was made by the Septennial Act 1715 for Parliament to dissolve automatically after seven, and later five years, in practice dissolution was always effected shortly before the end of the five-year period by means of a royal proclamation). Although Prime Minister David Cameron met the Queen on the day of the dissolution, the only business discussed was the calling of the new parliament, and not a request for a dissolution, as had happened at every such meeting historically, and the subsequent royal proclamation made on 30 March simply called for the holding of the next parliament (and therefore the holding of the general election).
Prior to the Fixed-term Parliaments Act 2011, each parliament would expire after a five-year term, as laid down in the Septennial Act 1715 (as amended by the Parliament Act 1911). This could, however, be overridden at a time of national emergency. The length of a parliament was extended on two occasions since 1911, once during each of the two World Wars. At any time the sovereign could dissolve Parliament and call a general election. In accordance with constitutional convention, the sovereign did not act independently, but at the request of the Prime Minister. Constitutional experts held that the monarch might refuse permission if a parliament had more than a year still to run and if another person could potentially command a majority in the House of Commons (see the Lascelles Principles), meaning that in practice a Prime Minister with a Commons majority and enjoying the support of his party had de facto authority to dissolve Parliament at a time of his choosing. Prior to 1918, it was the Cabinet who collectively sought permission from the monarch in order for Parliament to be dissolved. However, since 1918, the prime minister alone sought the permission of the sovereign.
Fixed term parliaments were introduced by the Fixed-term Parliaments Act 2011 following the Conservative – Liberal Democrat Coalition Agreement promulgated after the 2010 election, thereby abolishing the ability of the prime minister unilaterally to request an election prior to the expiry of the five-year term.