Enact a proceedure/regulation/law, (whichever is applicable) which triggers a by-election whenever an MP defects from the party under whose banner they were elected.
Why is this important?
In more recent times we have seen a relatively large number of MP's who were elected under Conservative, Labour, Liberal party etc banner/manifesto switch allegiance to other parties and still remain as MP's. Surely as those individuals were elected by the public to serve under a particular party manifesto it is wrong for them to switch to another party thereby denying their electorate of representation under the terms for which they were elected. Quite recently at least two sitting MP's have switched alegiance twice whilst holding onto their seats. This can not be right....can it?
It is clear that MP's will not always agree with the party line. In that case they can abstain from voting on a particular issue. However if they find that they fundamentally disagree with the party then surely the proceedure should be to trigger a by-election and seek re-election under the banner of their now preferred party?
The ability to 'jump ship', seemingly at an individuals whim, alters the balance in Parliament, does a disservice to the electorate and allows MP's to cross to 'the other side' with impunity. It needs to be stopped.