From that date EU workers moving to the UK will have to register, at least until a permanent post-Brexit immigration policy is put in place.
But Home Secretary Amber Rudd has sought to reassure business there will not be a "cliff edge" in terms of employing foreign workers after Brexit.
She said policy would be evidence-based and take into account economic impact.
The CBI said businesses "urgently" needed to know what EU migration would look like, both in any "transitional" period after March 2019 and beyond.