The optical properties of a photonic crystal slab made of periodic arrays of gold nanowires on top of different dielectric substrates, supporting and not supporting the guided modes are discussed. While only Rayleigh-type anomalies are observed for thin dielectric substrates, thicker waveguiding substrates demonstrate strong coupling phenomena. It is shown that the interaction between the quasiguided modes in the photonic crystal slab and the localised plasmons in metal nanostructures results in the formation of a strongly coupled waveguide-plasmon polariton. The formation of the new quasiparticle manifests itself in the strong anticrossing between quasiguided modes and plasmon resonances in the measured as well as calculated optical spectra. The effect opens new possibilities for photonic band gap engineering in metallic-dielectric photonic crystals.