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 ν-√ν regime in the plasma core, but it is close to the transition between the 1/ν and ν-√ν regimes. The neoclassical toroidal viscosity (NTV) torque in the 1/ν and ν-√ν regimes is calculated. The observed torque is of a magnitude in between that of the NTV torque in the 1/ν and ν-√ν regimes. The NTV torque in the ν-√ν 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.