  We calculate the non-reciprocal critical current and quantify the
supercurrent diode effect in Rashba superconductors with arbitrary disorder,
using the quasiclassical Eilenberger equation. The non-reciprocity is caused by
the helical superconducting state, which appears when both inversion and
time-reversal symmetries are broken. In the absence of disorder, we find a very
strong diode effect, with the non-reciprocity exceeding 40% at optimal
temperatures, magnetic fields and spin-orbit coupling. We establish that the
effect persists even in the presence of strong disorder. We show that the sign
of the diode effect changes as magnetic field and disorder are increased,
reflecting the changes in the nature of the helical state.
