A fuel supply system for a gas turbine engine including first and second positive displacement pumps operated simultaneously to supply fuel under pressure from a low pressure source, a combining spill valve controlling the output flows from the first and second pumps to combine the outputs of the first and second pumps for supply to a metering valve of the system, or to spill some or all of the output of one or both pumps back to the low pressure supply, a pressure raising and shut off valve arrangement downstream of the metering valve for isolating the fuel system from an associated engine until the fuel pressure upstream of the pressure raising and shut off valve arrangement exceeds a predetermined pressure, and, a control system dependent upon the position of the combining spill valve for reducing said predetermined pressure at which said pressure raising and shut off valve arrangement opens.