  We present a lattice calculation of the vector form factor of the pion for
two flavours of non-perturbatively O(a) improved Wilson fermions. For the
measurements we utilise the CLS ensembles which include various lattice
spacings and pion masses down to about 250 MeV. To obtain a fine momentum
resolution near zero momentum transfer (q^2) partially twisted boundary
conditions are employed using several twist angles. Due to the fine resolution
around q^2=0 we are able to determine the slope of the form factor and, in
turn, extract the charge radius of the pion without any model dependence. The
results for the form factor and the charge radius are then compared to chiral
perturbation theory and phenomenological models which are used to extrapolate
the results to the physical point.
