The Supreme Court’s new approach to mistake of law claims

Taxpayers often make mistakes. These mistakes can be a result of their own errors like inserting the wrong number in a self-assessment or failing to apply new rules brought in by the latest Finance Act. But mistakes can also arise when taxpayers – seemingly correctly – pay tax in accordance with the legal rules then in force but those rules later turn out to be unlawful. Perhaps the government implements law that is later found to be invalid or a legal principle is overturned after years of litigation and it becomes clear that tax never needed to be paid.

In these cases taxpayers will want to recover as much overpaid tax as possible. The...