In the case of a Prime Minister resigning, a general election should be called at the earliest opportunity after a new leader of the party is elected, to allow the general population to choose whether they still trust the current government or not.
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In the last few years 3 Prime Ministers have resigned and their successors chosen by their party and it has been business as usual despite public apathy and anger.
While I agree that a party is in the right to choose their own leader I believe that "business as usual" should only be the case if a Prime Minister resigns for health reasons.
For all other cases it is only right that the public choose whether they want a new leadership or not and not down to the current government.