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The optical and near‐infrared (near‐IR) late‐time spectra of the under‐luminous Type Ia supernova 2003hv are analysed with a code that computes the nebular emission from a supernova nebula. Synthetic spectra based on the classical explosion model W7 are unable to reproduce the large Fe  iii /Fe  ii  ratio and the low IR flux at ∼1 year after the explosion, although the optical spectrum of SN 2003hv is reproduced reasonably well for a supernova of luminosity intermediate between normal and subluminous (SN 1991bg‐like) ones. A possible solution is that the inner layers of the supernova ejecta ( v ≲ 8000 km s −1 ) contain less mass than predicted by classical explosion models like W7. If this inner region contains  ∼ 0.5   M ⊙   of material, as opposed to  ∼ 0.9   M ⊙   in Chandrasekhar‐mass models developed within the single‐degenerate scenario, the low density inhibits recombination, favouring the large Fe  iii /Fe  ii  ratio observed in the optical, and decreases the flux in the [Fe  ii ] lines which dominate the IR spectrum. The most likely scenario may be an explosion of a sub‐Chandrasekhar‐mass white dwarf. Alternatively, the violent/dynamical merger of two white dwarfs with the combined mass exceeding the Chandrasekhar limit also shows a reduced inner density.

The nebular spectrum of the Type Ia supernova 2003hv: evidence for a non‐standard event