We present a new determination of the interstellar 12CH+/13CH+ ratio in the solar neighbourhood, based on high spectral resolution CCD observations of the CH+ molecule in the direction of the star HD 26676 (d ≃ 150 pc). The measured absorption at λ4233 Å is one of the most intense 12CH+ lines detected so far (Wλ =23.5 mÅ), and is produced in the reflection nebula surrounding HD 26676. The corresponding 13CH+ absorption (Wλ = 0.37±0.l7 mÅ), observed for the first time in a star fainter than mυ = 6m, is detected at 3.6 level. After taking into account the saturation effects, we derive an abundance ratio of 12CH+/13CH+ = 77±36, where the error bar includes a conservative estimate of the uncertainty in the continuum level. Our result is only marginally consistent with the low values (12CH+/13CH+ ≃ 41) derived by Hawkins & Jura (1987) for two reflection nebulae (20 Tau, 23 Tau) in the same Galactic region as HD 26676, and therefore it does not support a decrease by a factor of 2 of the 12C/13C ratio since formation of the Sun. A modest decrease of this ratio in the solar vicinity, in agreement with the most recent models of Galactic chemical evolution, and similar to the one observed by Stahl et al. (1989) towards ζ Oph, is more consistent with our result.