  Topological superconductors (SCs) are novel phases of matter with nontrivial
bulk topology. They host at their boundaries and vortex cores zero-energy
Majorana bound states, potentially useful in fault-tolerant quantum
computation. Chiral SCs are particular examples of topological SCs with finite
angular momentum Cooper pairs circulating around a unique chiral axis, thus
spontaneously breaking time-reversal symmetry (TRS). They are rather scarce and
usually feature triplet pairing: best studied examples in bulk materials are
UPt3 and Sr2RuO4 proposed to be f-wave and p-wave SCs respectively, although
many open questions still remain. Chiral triplet SCs are, however,
topologically fragile with the gapless Majorana modes weakly protected against
symmetry preserving perturbations in contrast to chiral singlet SCs. Using muon
spin relaxation (muSR) measurements, here we report that the weakly correlated
pnictide compound LaPt3P has the two key features of a chiral SC: spontaneous
magnetic fields inside the superconducting state indicating broken TRS and low
temperature linear behaviour in the superfluid density indicating line nodes in
the order parameter. Using symmetry analysis, first principles band structure
calculation and mean-field theory, we unambiguously establish that the
superconducting ground state of LaPt3P is chiral d-wave singlet.
