  We give an integrability criterion for a projective limit of Banach
distributions on a Fr\'echet manifold which is a projective limit of Banach
manifolds. This leads to a result of integrability of projective limit of
involutive bundles on a projective sequence of Banach manifolds. This can be
seen as a version of Frobenius Theorem in Fr\'echet setting. As consequence, we
obtain a version of the third Lie theorem for a Fr\'echet-Lie group which is a
submersive projective limit of Banach Lie groups. We also give an application
to a sequence of prolongations of a Banach Lie algebroid.
