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By all-electron ab initio calculations, the layered polar semiconductor BiTeCl is shown to host giant bulk Rashba spin splitting, similar to the recently reported compound BiTeI. In both materials, the standard Rashba–Bychkov model is no longer applicable, because of huge band extrema shifts even in the absence of spin–orbit coupling and a strong momentum dependence of the Rashba coupling constant ( α _R ). By assuming α _R to be orbital dependent, a phenomenological extension of the Rashba–Bychkov model is proposed which explains the splitting behavior of states with small in-plane momentum.