Context: The determination of the lower-end of the initial mass function (IMF) provides strong constraints on star formation theories. Aims: We report here on a search for isolated planetary-mass objects in the 3 Myr-old star-forming region IC 348. Methods: Deep, narrowband CH4off and CH4on images were obtained with CFHT/WIRCam over 0.11 sq. deg. in the central part of IC 348 to identify young T-dwarfs from their 1.6 μm methane absorption bands. Results: We report three faint T-dwarf candidates with CH4on-CH4off colours >0.4 mag. Extinction was estimated for each candidate and lies in the range AV ~ 5-12 mag. Comparisons with T-dwarf spectral models, and colour/colour and colour/magnitude diagrams, reject two of the three candidates because of their extreme z'-J blueness. The one remaining object is not thought to be a foreground field dwarf because of a number density argument and also its strong extinction AV ~ 12 mag, or thought to be a background field T-dwarf which would be expected to be much fainter. Models and diagrams give this object a preliminary T6 spectral type. Conclusions: With a few Jupiter masses, the young T-dwarf candidate reported here is potentially amongst the youngest, lowest mass objects detected in a star-forming region so far. Its frequency is consistent with the extrapolation of current lognormal IMF estimates down to the planetary mass domain. Based on observations obtained with WIRCam, a joint project of CFHT, Taiwan, Korea, Canada, France, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the “University of Hawaii. Research supported by the Marie Curie Research Training Network CONSTELLATION” under grant no. MRTN-CT-2006-035890.