The non-linear ponderomotive force of laser-plasma interactions given by HORA (1985, Physics Fluids 28, 3706) is used as a basis for deriving the Lorentz and Gauge invariant expression. Hora's expression is reformulated as a 4-force in Minkowski space-time. The stress-energy tensor of this system is also derived. The Gauge invariance and Lorentz invariance are a direct result of the reformulation.