The Lascelles Principles was a constitutional convention in the United Kingdom between 1950 and 2011, under which the Sovereign could refuse a request from the Prime Minister to dissolve Parliament, if the existing Parliament was "still vital, viable, and capable of doing its job", if "a General Election would be detrimental to the national economy", and if the Sovereign "could rely on finding another prime minister who could govern for a reasonable period with a working majority in the House of Commons." The convention was rendered irrelevant in 2011, when the Sovereign's power to dissolve Parliament was removed by the Fixed-term Parliaments Act.
During public discussion in 1950 of the King's potential response to the slim majority of the newly re-elected Labour Party, the Lascelles Principles were formally stated in a letter by Sir Alan Lascelles, Private Secretary to King George VI, under the pseudonym "Senex" to the Editor of The Times, published 2 May 1950:
To the Editor of The Times
Sir, It is surely indisputable (and common sense) that a Prime Minister may ask--not demand--that his Sovereign will grant him a dissolution of Parliament; and that the Sovereign, if he so chooses, may refuse to grant this request. The problem of such a choice is entirely personal to the Sovereign, though he is, of course, free to seek informal advice from anybody whom he thinks fit to consult.
In so far as this matter can be publicly discussed, it can be properly assumed that no wise Sovereign—-that is, one who has at heart the true interest of the country, the constitution, and the Monarchy--would deny a dissolution to his Prime Minister unless he were satisfied that: (1) the existing Parliament was still vital, viable, and capable of doing its job; (2) a General Election would be detrimental to the national economy; (3) he could rely on finding another Prime Minister who could carry on his Government, for a reasonable period, with a working majority in the House of Commons. When Sir Patrick Duncan refused a dissolution to his Prime Minister in South Africa in 1939, all these conditions were satisfied: when Lord Byng did the same in Canada in 1926, they appeared to be, but in the event the third proved illusory.
I am, &c.,
—"Dissolution of Parliament: Factors in Crown's Choice", The Times, 2 May 1950, page 5
Thus, the letter asserted the constitutional power of the Sovereign to deny a dissolution, described the conditions for a valid exercise of that power, and referenced precedents when requests for parliamentary dissolution not exhibiting these conditions were refused by Governors-General of British Commonwealth countries, who represent the Sovereign.
- The Economist, 24 December 1994, page 32 (cited in British Government and the Constitution: Text and Materials, Colin Turpin and Adam Tomkins, 2007, page 364, ISBN 978-0-521-69029-4)