We derive group branching laws for formal characters of subgroups Hπ of GL(n) leaving invariant an arbitrary tensor T π of Young symmetry type π where π is an integer partition. The branchings GL(n) ↓ GL(n − 1) , GL(n) ↓ O(n) and GL(2n) ↓ Sp(2n) fixing a vector vi , a symmetric tensor gij = gji and an antisymmetric tensor fij = −fji , respectively, are obtained as special cases. All new branchings are governed by Schur function series obtained from plethysms of the Schur function sπ ≡ {π} by the basic M series of complete symmetric functions and the L = M −1 series of elementary symmetric functions. Our main technical tool is that of Hopf algebras, and our main result is the derivation of a coproduct for any Schur function series obtained by plethysm from another such series. Therefrom one easily obtains π -generalized Newell-Littlewood formulae, and the algebra of the formal group characters of these subgroups is established. Concrete examples and extensive tabulations are displayed for H 1 3 , H21 , and H3 , showing their involved and nontrivial representation theory. The nature of the subgroups is shown to be in general affine, and in some instances non reductive. We discuss the complexity of the coproduct formula and give a graphical notation to cope with it. We also discuss the way in which the group branching laws can be reinterpreted as twisted structures deformed by highly nontrivial 2-cocycles. The algebra of subgroup characters is identified as a cliffordization of the algebra of symmetric functions for GL(n) formal characters. Modification rules are beyond the scope of the present paper, but are briefly discussed. applications of Kronecker products is provided by [13]. A necessary concomitant of these techniques is the automation provided by a symbolic computer package such as SCHUR c [36]. Finally, some of these techniques have been found to generalize to the representation theory of non-compact groups [19,17,18] In a recent paper [10] the role of symmetric functions in relation to group representation theory has been re-considered from the viewpoint of the underlying Hopf algebraic structure. This structure is in fact well known in the combinatorial literature [34,33,31]. In [10] the formalism of branching rules was aligned with certain endomorphisms on the algebra of symmetric functions, called branching operators, derived from 1-cochains, for which the multiplicative cohomology of Sweedler [32] provides a natural analytical setting and classification. Standard branchings from generic symmetric functions to symmetric functions of orthogonal or symplectic type (the classic Newell-Littlewood theorems for the group reduction from GL(n) ↓ O(n) or GL(n) ↓ Sp(n) ) were found to be derived from certain 2-cocycles (for which associativity is guaranteed).This result then prompts the more general question of classifying arbitrary, non-cohomologous, 2cocycles, and the nature of any associated character theory and of the algebraic or group structures which might be entrained therewith. While an ab initio approa...