Abstract We fabricated high‐quality YBa 2 Cu 3 O 7‐δ /La 2/3 Ca 1/3 MnO 3 bilayers with 10 monolayers (∼ 12 nm) thin superconducting YBa 2 Cu 3 O 7‐δ (YBCO)layers on top of ferromagnetic La 2/3 Ca 1/3 MnO 3 (LCMO) layers with a thickness varied between 5 and 35 monolayers (∼ 2–13 nm). We measured the transport properties of the YBCO layers and the magnetic properties of the LCMO layers as a function of the LCMO layer thickness. We observed a smooth linear increase of the Curie temperature T C of the LCMO layer in the investigated thickness range, but a sudden strong decrease of the superconducting critical temperature T c of the YBCO layer at a critical LCMO thickness of ∼ 12 nm. Since no dramatic change of the structural quality of the YBCO layers is observed we conclude that the increasing magnetization strength in the LCMO layer is responsible for the T c suppression in the YBCO layer via proximity effect.