We present optical spectropolarimetry confirming Vermeulen et al.' recent discovery of a broad Ha emission line in BL Lacertae. This feature, which is present at about the same strength in spectra taken two days apart, is comparable in width (FWHM ≈ 4000 km s−1) and luminosity (LHα ∼ 1041 erg s−1) to the broad Hα lines of Seyfert type 1 galaxies. We also identify narrow lines of [N ii] λ6583, [O i] λ6300 and [Fe vii] λ6087. The continuum polarization was at a relatively low level ( ≈ 5 per cent) with no significant wavelength dependence. We estimate that the possible depolarizing effect of the line emission is too small to be clearly detected in our data. Simultaneous photometry shows that the source was not in an unusually faint state, although its V magnitudes were close to the lower limit of its typical variability range. Frequent monitoring during one night revealed a very rapid decrease in brightness by ≈0.1 mag in 30 min. We tentatively conclude that the broad Hα feature must have increased in luminosity by a factor ∼ 4 since earlier spectroscopic observations. While the beamed synchrotron continuum associated with the radio source can account for the observed equivalent width, the fact that the line has not previously been detected suggests either a sudden injection of cold gas into the radiation field or the strengthening of an additional ionizing radiation source. We show that the Hα emission could be powered by thermal radiation from an accretion disc without significantly affecting the shape or polarization of the optical continuum. The presence of a broad emission line at the redshift of the host galaxy demonstrates that the prototype BL Lac object is not a microlensed high-redshift quasar.