Leopold and Lilian initially planned to hold their official, civil marriage after the end of the war and the liberation of Belgium, but in the meantime, a secret religious marriage ceremony took place on 11 September 1941, in the chapel of Laeken Castle, in the presence of Queen Elisabeth, Henri Baels, and Cardinal van Roey, Archbishop of Mechelen and primate of Belgium.
Leopold dan Lilian awalnya berencana untuk melakukan pernikahan sipil resmi mereka setelah berakhirnya perang dan pembebasan Belgia, namun sementara itu, sebuah upacara pernikahan religius rahasia berlangsung pada tanggal 11 September 1941, di kapel Istana Laeken, di kehadiran Ratu Elisabeth, Henri Baels, dan Kardinal van Roey, Uskup Agung Mechelen dan primata Belgia.