We present a new, large sample of ultraluminous (L60>1011.4 L⊙ for H0 = 100 km s−1 Mpc−1) IRAS galaxies selected from the IRAS Faint Source Survey using a novel optical far-infrared colour selection criterion. Our technique proves to be very successful in selecting luminous galaxies, and we present data on 91 ultraluminous objects, of which 65 are newly discovered. Using our redshift spectra and information from the literature we find that at least 35 per cent of these objects are likely to contain an AGN-type central engine. We also find that purely IR-based techniques for determining the presence of an AGN in these objects are unreliable.