Unitals in with a large 2-point stabiliser

Let U be a unital embedded in the Desarguesian projective plane PG ( 2 , q 2 ) . Write M for the subgroup of PGL ( 3 , q 2 ) which preserves U . We show that U is classical if and only if U has two distinct points P , Q for which the stabiliser G = M P , Q has order q 2 − 1 .