We investigated the continuum absorption of water vapour diluted in nitrogen, oxygen and air in the range of the pure rotational band of the water molecule (50-500 cm-1). Spectra recordings were performed at room temperature with a Fourier transform spectrometer associated to a 151-m multipass gas cell located at the AILES beamline of SOLEIL synchrotron facility. The study includes the first laboratory measurements in the wide 90-330 cm-1 interval. Tests of the baseline stability, crucial for the continuum determination, are reported together with the expected pressure dependences of the continuum absorption, measured over different pressure ramps. Retrieved foreign-continuum cross-sections are found in good agreement with literature values available in the lower and upper parts of the studied frequency range. The reported results validate the MT_CKD foreign-continuum empirical model, widely used in atmospheric applications, even if some overestimation of the MT_CKD values is noted in the centre of the band where experimental data were absent.