  The precise determination of the CKM elements $V_{cb}$ and $V_{ub}$ is
crucial for any new physics analysis in the flavour sector. Their values can be
determined from several tree-level decays: $V_{cb}$ can be extracted from $B\to
D\ell\nu$ and $B\to D^*\ell\nu$ while $V_{ub}$ can be obtained from $B\to
\pi\ell\nu$, $B\to \rho\ell\nu$ and $B\to\tau\nu$. In addition, for both $V_{cb
}$ and $V_{ub}$ an inclusive determination is available. There is a long
lasting discrepancy between the inclusive and exclusive determinations which
recently even increased for $V_{cb}$ above the $3\;\sigma$ level. In this
article we study the possible effect of new physics on the inclusive and
exclusive determination of $V_{cb}$ and $V_{ub}$ in a model independent way. We
find that there is only one operator corresponding to a modified $W$ coupling
which can achieve this. However, respecting $SU(2)$ gauge invariance at the
high scale this would lead to very large violations of the $Z$ to $b\bar b$
coupling not compatible with experiment. Therefore, we conclude that a new
physics explanation of the difference between the inclusive and exclusive
determination of $V_{cb}$ and $V_{ub}$ is currently ruled out. Therefore, the
discrepancies must be due underestimated uncertainties in the theoretical
and/or the experimental analysis.
