Half of 18 satellites now in orbit have seen their clocks stop working.
Three are traditional rubidium devices; six are the more precise hydrogen maser instruments that were designed to give Galileo superior performance to the GPS network.
A decision must now be made about whether to suspend the launch of further spacecraft while the problem is investigated.
Prof Jan Woerner, the director general of the European Space Agency (ESA), told a meeting with reporters: “Everybody is raising this question: should we postpone the next launch until we find the root cause, or should we launch?
“You can give both answers at the same time. You can say we wait until we find the solution but that means if more clocks fail we will reduce the capability of Galileo. But if we launch we will at least maintain if not increase the [capability], but we may then take the risk that a systematic problem is not considered. We are right now in this discussion about what to do.”
Each Galileo satellite carries two rubidium and two hydrogen maser clocks. The multiple installation enables a satellite to keep working after an initial failure.
All 18 spacecraft currently in space continue to operate, but one of them is now down to just two clocks.
Most of the maser failures (5) have occurred on the satellites that were originally sent into orbit to validate the system, whereas all three rubidium stoppages are on the spacecraft that were subsequently launched to fill out the network.
Esa is also in contact with the Indian space agency which is using the same clocks in its sat-nav system. So far, the Indians have not experienced the same failures.
It is possible that the failures are linked to probable short circuits, and possibly a test performed on the ground.