  We present a detailed experimental and theoretical study on the relativistic
non-dipole effects in strong-field atomic ionisation by near-infrared
linearly-polarised few-cycle laser pulses in the intensity range 1014 -1015
W/cm2. We record high-resolution photoelectron momentum distributions of argon
using a reaction microscope and compare our measurements with a truly ab-initio
fully relativistic 3D model based on the time-dependent Dirac equation. We
observe counter-intuitive peak shifts of the transverse electron momentum
distribution in the direction opposite to that of laser propagation as a
function of laser intensity and demonstrate an excellent agreement between
experimental results and theoretical predictions.
