This paper presents the measurement of the relative width difference ΔΓ d /Γ d of the $$ {B}^0\hbox{-} {\overline{B}}^0 $$ system using the data collected by the ATLAS experiment at the LHC in pp collisions at $$ \sqrt{s}=7 $$ TeV and $$ \sqrt{s}=8 $$ TeV and corresponding to an integrated luminosity of 25.2 fb−1. The value of ΔΓ d /Γ d is obtained by comparing the decay-time distributions of B 0 → J/ψK S and B 0 → J/ψK *0(892) decays. The result is ΔΓ d /Γ d = (−0.1±1.1 (stat.)± 0.9 (syst.)) × 10−2. Currently, this is the most precise single measurement of ΔΓ d /Γ d . It agrees with the Standard Model prediction and the measurements by other experiments.