Abstract: The invention relates to a method of operating a gas turbine unit comprising a combustion chamber, a high pressure turbine driven by gas from the combustion chamber, and a low pressure turbine driven by gas from the high pressure turbine, means for controlling the gas supply from the high pressure turbine to the low pressure turbine, and a low pressure compressor driven by the low pressure turbine, and a high pressure compressor connected in series with the low pressure compressor and driven by the high pressure turbine for supplying pressurized air to the combustion chamber, the useful power of the gas turbine unit being adjustable by controlling the gas condition upstream of the low pressure turbine by the control means. Substantially all the air delivered to the combustion chamber from the compressors, is utilized for the combustion in the combustion chamber.