A fuel supply system for a gas turbine engine comprises a first positive displacement pump (16), a second positive displacement pump (27), said first and second positive displacement pumps (16, 27) being operable simultaneously for the supply of fuel from a low pressure source, and switching valve means (14) downstream of said first positive displacement pump (16) for changing a connection mode between said first and second positive displacement pumps (16, 27) between a series mode and a parallel mode.