The bathroom lines at the already crowded space shuttle and space station complex got a lot longer because of a flooded toilet.

One of two commodes aboard the international space station broke down, right in the middle of complicated robotic work being conducted by the two crews. The pump separator apparently flooded.

Mission Control advised the astronauts to hang an "out of service" sign on the toilet until it could be fixed.

In the meantime, the six space station residents had to get in line to use their one good toilet. And Endeavour's seven astronauts were restricted to the shuttle bathroom.

There have never been so many people - 13 - together in space.

The toilet repair work fell to Belgian Frank De Winne and American Michael Barratt, who had to don goggles, gloves and masks.

They ripped apart the compartment, working well into the evening. Mission Control finally instructed them to call it a day and resume the effort today.

Flight director Brian Smith declined to speculate whether overuse caused the toilet trouble.

"We don't yet know the extent of the problem," Mr Smith told reporters. "It may turn out to be of no consequence at all. It could turn out to be significant. It's too early to tell right now."

Teams of specialists in Houston and Moscow hurriedly convened to discuss the problem. The Russian-built, multimillion-dollar toilet flew up on a shuttle last November.

Mr Smith said there is no urgency to the bathroom situation, at least for now. But he said if the toilet remains out of action for several days, "then we'll readdress the situation and see what we have to do".