If an MP defects to another party, a by-election should be called immediately.

Background

A new law should be introduced to ensure constituents, voting for a candidate who represents their chosen party defects, there should be a by-election to confirm the constituency still want representation from this individual.

Additional details

Very often constituents vote according to party preferences and may indeed vote tactically to ensure certain parties do not win a seat. It is therefore incongruent for an MP to decide to change allegiance without support from her / his constituents. A by-election should be called immediately.