A gas turbine engine (20) comprising a variable geometry engine compressor (24), a variable geometry engine turbine (30) coupled to the engine compressor (24) and a combustor (28). The combustor (28) has an inlet (34) arranged to receive air from a first engine compressor outlet (34), and an outlet arranged to deliver combustion products to the engine turbine (30); a combustor bypass passage (50) having an inlet (53) arranged to receive air from a second engine compressor outlet (53), and an outlet (54) in fluid communication with a main fluid flow path downstream of a combustion zone (46, 48) of the combustor (28), and upstream of a turbine inlet (31); wherein the combustor bypass passage (50) comprises a bypass control valve (52) configured to selectively modulate the ratio of air flowing to the combustor inlet (34) to air flowing through the bypass passage (50).