It has been proved by Ziglin, for a large class of two-degree-of-freedom (2DOF) Hamiltonian systems, that transverse intersections of the invariant manifolds of saddle fixed points imply infinite branching of solutions in the complex time plane and the non-existence of a second analytic integral of the motion. Here, we follow a similar approach to show the existence of infinitely sheeted solutions for 2DOF. Hamiltonians which exhibit, upon perturbation, subharmonic bifurcations of resonant tori around an elliptic fixed point. Moreover, as shown recently, these Hamiltonian systems are non-integrable if their resonant tori form a dense set.