A strong toroidal rotation braking has been observed in plasmas with application of an n = 1 magnetic perturbation field on the JET tokamak. Calculation results from the momentum transport analysis show that the torque induced by the n = 1 perturbation field has a global profile. The maximal value of this torque is at the plasma core region (ρ < 0.4) and it is about half of the neutral beam injection torque. The calculation shows that the plasma is mainly in the regime in the plasma core, but it is close to the transition between the 1/ν and regimes. The neoclassical toroidal viscosity (NTV) torque in the 1/ν and regimes is calculated. The observed torque is of a magnitude in between that of the NTV torque in the 1/ν and regimes. The NTV torque in the regimes is enhanced using the Lagrangian variation of the magnetic field strength. However, it is still smaller than the observed torque by one order of magnitude.