The Lascelles Principles are a constitutional convention in the United Kingdom beginning in 1950, under which the sovereign can refuse a request from the prime minister to dissolve Parliament if three conditions are met: if the existing Parliament is 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". Wikipedia