We searched for planetary nebulae (PNe) in the nearby spiral galaxy M33 using the Wide Field Camera at the 2.54 m Isaac Newton Telescope of La Palma (Spain). Images in the [O iii] nebular line at lambda 5007Ä, in the Hα +[N ii] lines around 6560 Ä, and in the continuum around 5550Ä allowed us to detect a large number of emission line objects. Among them, 134 candidate PNe have been selected. In addition, we found 63 emission-line objects which have a non-negligible continuum emission, and for this reason they have been conservatively discarded from the group of candidate PNe. We have derived the PN luminosity function. It agrees very well with that of galaxies in the Local Group. The resulting distance to M33 is 840+/-90 kpc, in excellent agreement with the value obtained using Cepheids. Based on observations obtained at the 2.5m~INT telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias.