A new ab initio method for fully relativistic (four-component Dirac many-body formalism) correlated electronic structure calculations, which makes explicit reference to valence electrons only, is presented for atomic and molecular processes in which the core electrons remain inert; The method exactly reproduces all properties (wave functions, energies, etc.) of an all electron. calculation with frozen core wave functions and thus provides a,great computational advantage. The method is based on constructing a valence Hamiltonian which includes a one-electron effective potential for the core-valence interactions. This interaction potential, which needs to be calculated once and for all, is exact for a chosen basis since it is constructed (all constituent local and nonlocal interactions) in matrix form in the current basis. This potential is used subsequently in valence-electron-only calculations, based on a novel algorithm, by constructing explicitly valence orbitals orthogonal to the core orbitals as linear combinations only among themselves. It has no adjustable parameters and no restrictions on the choice of basis sets. It yields considerable savings of computer time and space over the corresponding frozen core all-electron calculations. Dirac-Fock SCF calculations are presented for the ground states of closed- and open-shell atoms Li, Be, B, C, N, O, F, F-, Ne, Na+, Na, Al, and S for demonstration.