The invention relates to a process for the start-up of an autothermal reformer, wherein syngas is produced in the autothermal reformer during start-up through steam reforming. To facilitate autoignition in the autothermal reformer reactor of the autothermal reformer, the reformed syngas is recycled to an upstream section of the autothermal reformer reactor and is mixed with process steam and a hydrocarbon containing process stream. As soon as a minimum hydrogen threshold concentration at the upstream section of the autothermal reformer reactor is reached in the mixed process stream, oxygen is added to the burner of the ATR reactor to obtain autoignition of the mixed process stream. Due to the process of the invention, an external hydrogen source for facilitating autoignition of the mixed stream can be omitted. The invention further relates to a plant configured to carry out the process of the invention.