A gas turbine engine comprising: a combustor including fuel injectors; and a fuel supply system for delivering fuel to said fuel injectors, said fuel supply comprising: a fuel tank; a first fuel pump; a first oxygen removal unit, said first fuel pump connected to deliver fuel from said fuel tank into said first oxygen removal unit; characterised by a second oxygen removal unit; a valve downstream of said first oxygen removal unit; wherein the valve is connected to a second pump, wherein said second pump is connected to deliver fuel into the second oxygen removal unit and said valve being operable to selectively deliver fuel back to said fuel tank in a bypass position, or downstream to the fuel injectors, via the second oxygen removal unit, in a use position; and a controller programmed to control said valve and said first fuel pump, said controller controlling said first fuel pump to deliver the fuel from the fuel tank to the first oxygen removal unit and maintaining said valve in said bypass position if the gas turbine engine is not operating, so as to deliver fuel back to the fuel tank, and moving said valve to said use positon when the gas turbine engine is operating. a combustor including fuel injectors; and a fuel supply system for delivering fuel to said fuel injectors, said fuel supply comprising: a fuel tank; a first fuel pump; a first oxygen removal unit, said first fuel pump connected to deliver fuel from said fuel tank into said first oxygen removal unit; characterised by a second oxygen removal unit; a valve downstream of said first oxygen removal unit; wherein the valve is connected to a second pump, wherein said second pump is connected to deliver fuel into the second oxygen removal unit and said valve being operable to selectively deliver fuel back to said fuel tank in a bypass position, or downstream to the fuel injectors, via the second oxygen removal unit, in a use position; and a controller programmed to control said valve and said first fuel pump, said controller controlling said first fuel pump to deliver the fuel from the fuel tank to the first oxygen removal unit and maintaining said valve in said bypass position if the gas turbine engine is not operating, so as to deliver fuel back to the fuel tank, and moving said valve to said use positon when the gas turbine engine is operating. a fuel tank; a first fuel pump; a first oxygen removal unit, said first fuel pump connected to deliver fuel from said fuel tank into said first oxygen removal unit; characterised by a second oxygen removal unit; a valve downstream of said first oxygen removal unit; wherein the valve is connected to a second pump, wherein said second pump is connected to deliver fuel into the second oxygen removal unit and said valve being operable to selectively deliver fuel back to said fuel tank in a bypass position, or downstream to the fuel injectors, via the second oxygen removal unit, in a use position; and a controller programmed to control said valve and said first fuel pump, said controller controlling said first fuel pump to deliver the fuel from the fuel tank to the first oxygen removal unit and maintaining said valve in said bypass position if the gas turbine engine is not operating, so as to deliver fuel back to the fuel tank, and moving said valve to said use positon when the gas turbine engine is operating.