The Cabinet Manual stresses that the monarch should not be exposed to political decisions and "it remains a matter for the Prime Minister, as the Sovereign's principal adviser, to judge the appropriate time at which to resign".
[2] During public discussion of George VI's potential response to the outcome of the 1950 general election, which returned a very slim Labour Party majority in the House of Commons, the Lascelles Principles were formally stated in a letter to the Editor of The Times, written by the King’s Private Secretary Sir Alan Lascelles, and published on 2 May 1950, under the pseudonym "Senex": 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.
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.
[3] Historian Peter Hennessy stated in 1994 that the second of the three conditions had since been "dropped from the canon", being no longer included in internal Cabinet Office guidance.