The Theseus was the Bristol Aeroplane Company's first attempt at a gas-turbine engine design, a turboprop that delivered just over 2,000 hp (1,500 kW). A novel feature was the use of a heat exchanger to transfer waste heat from the exhaust to the compressor exit. The engine was soon superseded by the Proteus design with more power, and the only extended use of the engine was in two Handley Page Hermes 5 development aircraft.
Following 156 hours of ground runs and the receipt of a test certificate from the Ministry of Supply on 28 January 1947, two Theseus engines were fitted in the outer positions of a four-engined Avro Lincoln for air tests. After ground and taxying test the Lincoln first flew on 17 February 1947.