The inclusive branching ratio for the process 
$\rm b\rightarrow\tau^-\bar{\nu}_\tau X$ 
has been measured using hadronic Z decays collected by the OPAL experiment
at LEP in the years 1992-2000. The result is:
$$\mathrm{BR}(b\rightarrow\tau^-\bar{\nu}_\tau X) = (2.78\pm 0.18 \pm 0.51) \%$$
This measurement is consistent with the Standard Model expectation and 
puts a constraint of 
$$\tan\beta / M_{\rm H}^{\pm} < 0.53\ {\rm GeV}^{-1}$$
at the $95\%$ confidence level on Type II Two Higgs Doublet Models.