Hold a by-election whenever a sitting MP changes political party.

Background

People vote for an MP because they believe in a particular political party or manifesto, if that MP then changes their party they should stand again in a by-election. This would ensure that the voters are represented by the party they want rather than by the whim of a particular MP.

Additional details

An MP voted in on a particular manifesto/political party who then goes on to change their allegiance to another party is no longer representative of the voters in their constituency.