A time-dependent, flavour-tagged measurement of CP violation is performed with B0→ D+D− and Bs0→Ds+Ds− decays, using data collected by the LHCb detector in proton-proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 6 fb−1. In B0→ D+D− decays the CP-violation parameters are measured to be (Formula presented.) In Bs0→Ds+Ds− decays the CP-violating parameter formulation in terms of ϕs and |λ| results in (Formula presented.) These results represent the most precise single measurement of the CP-violation parameters in their respective channels. For the first time in a single measurement, CP symmetry is observed to be violated in B0→ D+D− decays with a significance exceeding six standard deviations.