The invention relates to a process for the treatment of heavy petroleum feedstocks in order to produce a gas oil fraction having a sulphur content of less than 50 ppm and generally 10 ppm which comprises the following stages: e) mild hydrocracking in a catalyst fixed bed, f) separation of the hydrogen sulphide, of a distillate cut including a gas oil fraction and of a fraction which is heavier than the gas oil, g) hydrotreating, by contact with at least one catalyst, of at least a portion of the distillate cut obtained in stage b) including the gas oil fraction, as a mixture with a feedstock resulting from a crude or refined renewable source, h) separation of a gas oil cut comprising less than 50 ppm of sulphur. Advantageously, the heavy fraction is sent for catalytic cracking. Preferably, the process is carried out with makeup hydrogen introduced in stage c) and very advantageously all the makeup hydrogen for the process is introduced in stage c). The invention also relates to a plant which can be used for implementing this process.