The magnitude of the CKM matrix element |Vub| is determined by measuring the inclusive charmless semileptonic branching fraction of beauty hadrons at OPAL based on b→Xuℓν event topology and kinematics. This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays. We measure Br( b→Xuℓν) to be (1.63±0.53+0.55−0.62)×10−3. The first uncertainty is the statistical error and the second is the systematic error. From this analysis, |Vub| is determined to be:
|Vub| = (4.00 ± 0.65(stat)+0.67−0.76(sys) ± 0.19(HQE)) × 10−3.
The last error represents the theoretical uncertainties related to the extraction of |Vub| from Br( b→Xuℓν) using the Heavy Quark Expansion.