Abstract: A gas turbine engine combustion chamber which has primary, secondary and tertiary combustion zones in flow series has a secondary fuel and air mixing duct and a tertiary fuel and air mixing duct. The secondary mixing duct has a secondary air intake at its upstream end and the tertiary mixing duct has a tertiary air intake at its upstream end. The tertiary air intake is arranged adjacent to the secondary air intake. A combined secondary and tertiary fuel system is provided to supply fuel to the secondary and tertiary mixing ducts. The fuel system comprises a manifold arranged adjacent to the secondary mixing duct but is spaced from the tertiary mixing duct by the secondary mixing duct. The manifold has apertures to direct fuel towards the tertiary air intake across the secondary air intake. Variations in fuel pressure cause the fuel to flow into the secondary air intake or the tertiary air intake.