  One of the main goals of Cosmology is to search for the imprint of primordial
gravitational waves in the CMB polarisation field, to probe inflationary
theories. One of the obstacles toward the detection of the primordial signal is
to extract the B-mode polarisation from astrophysical contaminations. We
present a complete analysis of extragalactic foreground contamination due to
polarised emission of radio and dusty star-forming galaxies. We update or use
up-to-date models that are validated using the most recent measurements. We
predict the flux limit (confusion noise) for the future CMB space or balloon
experiments (IDS, PIPER, SPIDER, LiteBIRD, PICO), as well as ground-based
experiments (C-BASS, NEXT-BASS, QUIJOTE, AdvACTPOL, BICEP3+Keck, BICEPArray,
CLASS, SO, SPT3G, S4). Telescope aperture size (and frequency) is the main
characteristic impacting the level of confusion noise. Using the flux limits
and assuming constant polarisation fractions for radio and dusty galaxies, we
compute the B-mode power spectra of the three extragalactic foregrounds (radio
source shot noise, dusty galaxy shot noise and clustering), discuss their
relative levels and compare their amplitudes to that of the primordial tensor
modes parametrized by the tensor-to-scalar ratio r. At the reionization bump
(l=5), contamination by extragalactic foregrounds is negligible. At the
recombination peak (l=80), while the contamination is much lower than the
targeted sensitivity on r for large-aperture telescopes, it is at comparable
level for some of the medium- and small-aperture telescope experiments. For
example, the contamination is at the level of the 68 per cent confidence level
uncertainty on the primordial r for the LiteBIRD and PICO space experiments.
Finally we also provide some useful unit conversion factors and give some
predictions for the SPICA B-BOP experiment. Abridged
