We describe the scientific case and design challenges for an innovative large submillimetre telescope (LST). LST is proposed to have a diameter of at least 30m, operate in the wavelength range from 0.2 - 1mm and will have a wide field-of-view. The submillimetre region allows us to probe objects during formation - i.e. the earliest evolutionary stages of galaxies, stars and planets. Being so close to the peak of the cosmic far-IR/submm background and the emission from proto-stellar cores, the 200μm atmospheric window gives access to unique science. The key advantage of LST over other facilities will be in terms of addressing astronomical questions requiring large fields and good angular resolution: such as surveys of entire giant molecular clouds and fields of dusty galaxies at early epochs. For example, such a telescope would resolve all protoplanetary disks out to 100pc, revolutionising our knowledge of star and planet formation, and detect tens of millions of dusty high-redshift galaxies yielding information on formation and evolution of the early universe. Equipped with a state-of-the-art large format bolometer camera, LST would offer a survey (mapping) speed up to 5,000 times that of ALMA, arcsecond resolution at the shortest wavelengths, point-source sensitivities at least 25 times better than any planned submm facility, and confusion limits 10 times lower than any existing single-aperture telescope. The wide-field-of-view and superb image fidelity available from a single-dish will be perfect for large-scale surveys of the submillimetre sky - an ideal complement to new generation interferometers such as ALMA.