Governments are elected based on a political party's manifesto to the British public. A bond of trust is therefore made to carry out those pledges, if they are directly broken so is the bond, rendering the GE result fraudulent. At this point the Opposition should have the power to call a fresh GE.
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The current government made a pledge in their manifesto stating: "We will not raise the rate of income tax, VAT or National Insurance". They have now announced plans to increase National Insurance in direct contravention of a promise made to the public in their manifesto - this misled the electorate and their position is therefore untenable.