The Lascelles Principles are a constitutional convention in the United Kingdom describing the circumstances under which a monarch may refuse a request from a Prime Minister for the dissolution of Parliament. The Lascelles principles are that the monarch can refuse a dissolution if "the existing Parliament was still vital, viable, and capable of doing its job" or if the monarch "could rely of finding another prime minister who could govern for a reasonable period with a working majority in the House of Commons."

The Lascelles Principles are rather notable in that their formal statement was not incorporated in any governmental document, but rather were stated in the form of a letter in 1951 to the editor to The Times by Sir Alan Lascelles writing under the pseudonym Senex.

They have never been applied in the United Kingdom since they were written, since no monarch has refused a dissolution since then.