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In this study multi-aperture photo electric photometry in the five passbands UBVRI for 171 and 46 galaxies in the Virgo and Fornax clusters, respectively, are presented. New radial velocities and H I profiles for 17 galaxies in Fornax have been obtained. Total magnitudes and colours are derived for around 800 individual measurements using standard growth curves in B, U-B and B-V. Total colours V-R and R-I are derived from a set of synthetic colour growth curves. The resulting total B magnitudes have an internal error of 0.09 mag, the U-B colours have an internal error of 0.03 mag, and in B-V V-R and R-I the error is 0.02 mag.

The derived magnitudes of a nearly complete sample (∼90%) of all Virgo and Fornax spirals with i >/= 45 degrees (as listed in the Virgo and Fornax cluster catalogues) lend themselves for a detailed study of the luminosity-line width relation (Tully-Fisher relation) in UBVRI. In particular the effects of different parameter sets (line widths, axial ratios, morphological types) and corrections on the luminosity-lkine width relation are investigated. Furthermore, the dependence on morphological galaxy type is examined, new internal absorption corrections are derived and the accuracy of the luminosity-line width method in different passbands is studied. The data are used to determine a distance of the spiral galaxies of the Fornax cluster relative to the Virgo spirals.

The results are as follows:

The slope of the luminosity-line width relation in B is -6.50 ± 0.36 and depends on the line width correction. Differences of up to 0.2 mag in the intercept of each cluster are found when using different input parameter; changes due to different corrections apply to both clusters simultaneously, and have little effect on the relative distance. Subsamples as to inclination or galaxy type affect the relative distance only marginally, while a restriction in apparent magnitude results in a change of the slope and the intercept of each cluster and of the relative distance. A limitation in line width affects Fornax more than Virgo, because the scatter in the luminosity-line width relation of the Fornax cluster changes strongly with line widths (or magnitude).

The luminosity-line width relation s in other passbands show the same scatter for Virgo (about 0.64 mag); in Fornax the scatter varies with wavelength and is largest in the I-band. The slope is dependent on wavelength and approaches an asymptotic value of about -8.5 at long wavelengths. The mean relative distance modulus over all passbands is m_F - m_V = -0.40 ± 0.10 mag.

The variation of the inclination dependent part of the internal absorption with wavelength is investigated. The colours show a dependence on axial ratio only for Sb to Scd galaxies. The resulting internal absorption with respect to the RC3 B-band absorption for these galaxy types is 116% in the U-band, 82% in V, 74% in R and 63% in I. Furthermore, a new absorption law of A_B = 1.7 log R was found for Sb to Scd galaxies (as compared with the RC3 law of A_B = 1.47 log R), though the present sample is too small for the result to be reliable.

The potential of a luminosity-colour relation is inspected. However, error and scatter are much higher than in the luminosity-line width relation. The resulting relative distance of Fornax and Virgo is consistent within the errors with the distance found using the luminosity-line width relation.

The spiral galaxies of the Fornax cluster are found to be closer than the Virgo spirals by 0.40 mag. The resulting peculiar velocity of Fornax relative to Virgo lies between 150 km s^-1 and 300 km s^-1, depending on the virgocentric infall model, and points away from the Local Group. The division of Virgo into subclusters and the effect of H I stripping are discussed.