A gas turbine engine cleaning system for cleaning gas paths in an engine core of a gas turbine engine, the gas turbine engine comprising an air intake (12, 212) and a propulsive fan that generates a core airflow and a bypass airflow, the engine core including in axial flow series, a front end, a low pressure compressor, a high pressure compressor, combustion equipment, a high pressure turbine,a low pressure turbine (19, a core exhaust nozzle and a rear end, a nacelle (21, 221) surrounding the gas turbine engine and defining a bypass duct (22, 222) and a bypass exhaust nozzle, the gas turbine engine cleaning system comprising: a source of an engine cleaning liquid; an engine cleaning mist forming unit that is configured to vapourise the engine cleaning liquid to form an engine cleaning mist and to deliver the engine cleaning mist into the engine core of the gas turbine engine; at least one delivery device is placed in front of the engine core near the front end and is configured to deliver the engine cleaning liquid to the engine cleaning mist forming unit; characterised in that the at least one delivery device is configured to be inserted from the rear end of the gas turbine engine through the bypass duct (22, 222) and into the engine core; the gas turbine engine cleaning system comprises a pump configured to draw the engine cleaning mist through the engine core to clean the gas paths within the engine core; the gas turbine engine cleaning system comprises a mist collecting arrangement comprising a condensing chamber, a tooling that forms a sealed interface with the engine core at the rear end, and a conduit that is connected to the tooling and configured to deliver the collected engine cleaning mist to the condensing chamber, wherein the pump is disposed in the conduit, and the mist collecting arrangement is configured to collect the engine cleaning mist that has passed through the engine core drawn by the pump and condense the collected engine cleaning mist in the condensing chamber; and the gas turbine engine is not operational during cleaning, and the cleaning system does not need to run a starter motor and does not require the compressor of the gas turbine engine to rotate during cleaning. a source of an engine cleaning liquid; an engine cleaning mist forming unit that is configured to vapourise the engine cleaning liquid to form an engine cleaning mist and to deliver the engine cleaning mist into the engine core of the gas turbine engine; at least one delivery device is placed in front of the engine core near the front end and is configured to deliver the engine cleaning liquid to the engine cleaning mist forming unit; characterised in that the at least one delivery device is configured to be inserted from the rear end of the gas turbine engine through the bypass duct (22, 222) and into the engine core; the gas turbine engine cleaning system comprises a pump configured to draw the engine cleaning mist through the engine core to clean the gas paths within the engine core; the gas turbine engine cleaning system comprises a mist collecting arrangement comprising a condensing chamber, a tooling that forms a sealed interface with the engine core at the rear end, and a conduit that is connected to the tooling and configured to deliver the collected engine cleaning mist to the condensing chamber, wherein the pump is disposed in the conduit, and the mist collecting arrangement is configured to collect the engine cleaning mist that has passed through the engine core drawn by the pump and condense the collected engine cleaning mist in the condensing chamber; and the gas turbine engine is not operational during cleaning, and the cleaning system does not need to run a starter motor and does not require the compressor of the gas turbine engine to rotate during cleaning.