  We calculate, in the continuum limit of quenched lattice QCD, the form factor
that enters in the decay rate of the semileptonic decay B --> D l nu. Making
use of the step scaling method (SSM), previously introduced to handle two scale
problems in lattice QCD, and of flavour twisted boundary conditions we extract
G(w) at finite momentum transfer and at the physical values of the heavy quark
masses. Our results can be used in order to extract the CKM matrix element Vcb
by the experimental decay rate without model dependent extrapolations.
