Prominent enhancement on the radiative branching ratio of the doubly excited resonant states due to the Breit interaction induced ion-core energy inversion

•A new mechanism that the Breit interaction effects significantly in the highly charged Li- and B-like ions are studied, the strongly correlated many-body wavefunctions of the doubly excited resonant states [1s2p1/22p3/2]3/2 (or [1s(2s2)2p1/22p3/2]3/2) 4P3/2 and 2D3/2 are drastically influenced by the Breit interaction.•The strong variation of wavefunctions results in a prominent change of the two dominant transitions 2p3/2–1 s and 2p1/2–1 s of the doubly excited states. The variation of the transitions can be observed in experiment by measuring the branching ratio of the doubly excited Li-like or B-like ions.•The variation of the wavefunctions is due to the ion-core energy inversion induced by the Breit interaction. Based on the mechanism, we propose an alternative experimental determination of the fine structure splitting of [1s2p1/2]0,1 (or [1s(2s2)2p1/2]0,1) in heavy He-like (or Be-like) ions from the branching ratio of the doubly excited Li-like (or B-like) ions.•This work extends the conventional wisdom on the role of the relativistic effects in the electric dipole radiative transitions of highly charged ions, which could be useful for future precise high-temperature plasma diagnostics.