In a fuel supply system for an internal combustion engine having two separate storage containers for liquid fuels, both connected to a first controllable valve which is connected, via a connecting line including a fuel pump to an inlet of a second controllable valve having two outlets in communication by separate fuel lines with a fuel injection nozzle of the internal combustion engine, each of the two separate fuel lines includes a fuel pressure regulator, one being in communication with one and the other with the other of the two separate fuel storage containers for returning exess fuel to the fuel storage container from which fuel is being supplied to the fuel injection nozzle.