HOUSTON (Reuters) -- NASA on Tuesday put off its next space shuttle mission until July at the earliest to replace fuel sensors in the shuttle's external fuel tank that could give faulty readings.

The delay in getting the shuttle fleet back into the air after it was grounded last July could further delay the multination orbital construction of the International Space Station, which depends on the U.S. space shuttles.

Shuttle program manager Wayne Hale said replacing the sensors meant NASA would not be able to make its original launch window scheduled from May 10.

The sensors ensure the shuttle's main engines shut down before they drain the liquid oxygen and liquid hydrogen propellants from the fuel tank.

Faulty sensors could cause the engines to shut down before the shuttle is in orbit, leaving the ship unable to reach its intended orbit or forcing a risky and unprecedented emergency landing.

Or, they could fail to cut off the engines if the shuttle actually runs out of fuel due to a leak or other problem and possibly trigger a catastrophic explosion. "This is a life-or-death kind of situation," Hale said. "You want to shut the engines down if you're running out of gas, but you don't want to shut the engines down early if you're not running out of gas. Either way you can get in trouble."

The next opportunity to launch Discovery to the International Space Station will be between July 1 and 19. Despite the delay, Hale said he is optimistic NASA will be able to fly three shuttle missions this year.

The U.S. shuttle fleet, which is scheduled for retirement in 2010, has been grounded since last July, when insulation foam broke off the external fuel tank of the shuttle Discovery as it launched from Cape Canaveral in Florida.

Falling insulation foam was the same problem that doomed the shuttle Columbia in 2003, knocking a hole in its wing and allowing superheated atmospheric gases to tear the shuttle apart when it re-entered Earth's atmosphere at the end of its mission. All seven crew members on board were killed.

The recurrence of the foam problem during Discovery's mission was a significant blow to NASA, which had spent $1 billion trying to fix the problem.

NASA has some experience with faulty fuel sensors. A sensor problem delayed shuttle Discovery's launch last year. Hale said he did not know if the sensor problem that will delay Discovery's launch this year is related to last year's problem.

Construction of the space station has been delayed by the Columbia accident. The shuttle is the only spacecraft with a cargo hold capable of carrying the station's larger modules.