Pronaos, a two metre balloonborne submillimetre telescope, supported by CNES and designed by a French collaboration between CNRS institutes and CNES-CST is presented. This project is now approved and in a preliminary definition phase B. It is intended to provide a facility for long duration flights (more than 20 hours) for photometry and spectroscopy in the spectral range 80 micrometres to 1.5 millimetre. Two focal plane instrumentations are considered : 1) a multiband photometre device using He3 cooled bolometres and He4 cooled filters. 2) a submillimetre heterodyne spectrometre. The two-metre Cassegrain telescope, with a lightweight primary mirror has an equivalent focal ratio F/10. The telescope is pointed (accuracy of 5"rms) with a two stages stabilisation having a two axis sys-tem : 1) the azimuth orientation of the gondola provides from the decoupling with the balloon (ac-curacy of a few arc-minutes) and 2) an elevation/ cross-elevation gimbal.