  We prove that any regular integral invariant of volume-preserving
transformations is equivalent to the helicity. Specifically, given a functional
$\mathcal I$ defined on exact divergence-free vector fields of class $C^1$ on a
compact 3-manifold that is associated with a well-behaved integral kernel, we
prove that $\mathcal I$ is invariant under arbitrary volume-preserving
diffeomorphisms if and only if it is a function of the helicity.
