  Measurable amounts of Be could have been synthesised primordially if the
Universe were non-homogeneous or in the presence of late decaying relic
particles. We investigate the Be abundance in the extremely metal-poor star
2MASS J1808-5104 ([Fe/H]=--3.84) with the aim of constraining inhomogeneities
or the presence of late decaying particles. High resolution, high
signal-to-noise ratio UV spectra were acquired at ESO with the Kueyen 8.2 m
telescope and the UVES spectrograph. Abundances were derived using several
model atmospheres and spectral synthesis code. We measured log(Be/H) = -14.3
from a spectrum synthesis of the region of the Be line. Using a conservative
approach, however we adopted an upper limit two times higher, i.e. log(Be/H) <
-14.0. We measured the O abundance from UV OH lines and find [O/H]=--3.46 after
a 3D correction. Our observation reinforces the existing upper limit on
primordial Be. There is no observational indication for a primordial production
of Be. This places strong constraints on the properties of putative relic
particles. This result also supports the hypothesis of a homogeneous Universe,
at the time of nucleosynthesis. Surprisingly, our upper limit of the Be
abundance is well below the Be measurements in stars of similar [O/H]. This may
be evidence that the Be-O relation breaks down in the early Galaxy, perhaps due
to the escape of spallation products from the gas clouds in which stars such as
2MASS J1808-5104 have formed.
