  We measure a large valley-orbit splitting for shallow isolated phosphorus
donors in a silicon gated nanowire. This splitting is close to the bulk value
and well above previous reports in silicon nanostructures. It was determined
using a double dopant transport spectroscopy which eliminates artifacts induced
by the environment. Quantitative simulations taking into account the position
of the donors with respect to the Si/SiO$_2$ interface and electric field in
the wire show that the values found are consistent with the device geometry.
