Abstract <p>The Van der Waals effect (VW) in ³⁹K atomic vapours was experimentally studied using a nanocell made of technical sapphire. At distances of less than 100 nm from the sapphire surface, a strong broadening of atomic transitions and their frequency shifts to the low-frequency region of the spectrum occur. The method of selective reflection (SR) of laser radiation from the sapphire window-atomic vapours boundary was used as a sub-Doppler method, which made it possible to measure the VW interaction coefficient <i>C</i>₃ for the potassium atoms. It is shown that a change in the nanocell thickness from 100 to 50 nm results in a decrease in <i>C</i>₃, as calculated from the “red” VW shift, i.e., the so-called “retardation” of the VW effect, which was predicted in theoretical works and experimentally observed. The obtained results are important for the development of miniature submicron devices containing atomic vapours.</p>