Aims. A proper determination of the abundance gradient in the Milky Way requires the observation of objects at large galactiocentric distances. With this aim, we are exploring the planetary nebula population towards the Galactic anticentre. In this article, the discovery and physico-chemical study of a new planetary nebula towards the anticentre direction, IPHASX J052531.19+281945.1 (PNG 178.1-04.0), is presented. Methods. The planetary nebula was discovered from the IPHAS survey. Long-slit follow-up spectroscopy was carried out to confirm its planetary nebula nature and to calculate its physical and chemical characteristics. Results. The newly discovered planetary nebula turned out to be located at a very large galactocentric distance (DGC = 20.8 ± 3.8 kpc), larger than any previously known planetary nebula with measured abundances. Its relatively high oxygen abundance (12+log(O/H) = 8.36± 0.03) supports a flattening of the Galactic abundance gradient at large galactocentric distances rather than a linearly decreasing gradient.