Alexander v Rayson  1 K.B. 169, at p.182:
It is settled law that an agreement to do an act that is illegal or immoral or contrary to public policy, or to do any act for a consideration that is illegal or immoral or contrary to public policy, is unlawful and therefore void. But it often happens that an agreement which in itself is not unlawful is made with the intention of one or both parties to make use of the subject-matter for an unlawful purpose, that is to say, a purpose that is illegal, immoral or contrary to public policy. … In such a case any party to the agreement who had the unlawful intention is precluded from suing upon it. Ex turpi causa non oritur actio. The action does not lie because the court will not lend its help to such a plaintiff.