Ab initio multiconfiguration calculations are performed for the oscillator strengths of the λ= 4238, 4262 and 4297 Å Tc i resonance lines of astrophysical interest. Electron correlation is treated through multiconfiguration expansions built from elaborate correlation models, while relativistic effects are introduced in the perturbation Breit–Pauli approximation or in the multiconfiguration Dirac–Fock–Breit variational approach. The calculated gf-values are sensitively lower (≈30 per cent) than the values obtained with the pseudo-relativistic Hartree–Fock wavefunctions calculated from a parametric analysis of Tc i by Palmeri & Wyart. The inclusion of a core-polarization potential in the latter approach confirms the present ab initio results when different ionic cores are used for the different transition arrays. The strong lines of Tc i are revisited adopting this model, giving rise to a systematic reduction in the oscillator strength scale due to core polarization. The astrophysical implications are discussed.