We report the discovery of correlated modulation of X-ray and radio emission properties of the gamma-ray binary LSI +61 303 on a superorbital 4.6 year time scale. Using systematic multi-year monitoring of the system by RXTE we show that the orbital phase of X-ray flares from the source varies from φX ≃ 0.35 to φX ≃ 0.75 on the superorbital time scale. Simultaneous radio observations show that periodic radio flares always lag the X-ray flare by ΔφX-R ≃ 0.2. We propose that the constant phase lag corresponds to the time of flight of the high-energy particle filled plasma blobs from inside the binary to the radio emission region at the distance ∼ 10 times the binary separation distance. We put forward a hypothesis that the X-ray bursts correspond to the moments of formation of plasma blobs inside the binary system. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial- ShareAlike Licence.