The process e+e−→W+W−γ is analysed using the data collected with the L3 detector at LEP at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.8 pb−1. Based on a sample of 42 selected W+W− candidates containing an isolated hard photon, the W+W−γ cross section, defined within phase-space cuts, is measured to be: σWWγ=290±80±16 fb, consistent with the Standard Model expectation. Including the process e+e-→vvγγ , limits are derived on anomalous contributions to the Standard Model quartic vertices W+W−γγ and W+W−Zγ at 95% CL: -0.043 GeV-2 < αo/Δ2< 0.043 GeV-2, -0.08 GeV <αo/Δ2 < 0.13GeV2, -0.041 GeV-2 < αo/Δ2 < 0.37 GeV-2