The falling cover damaged two heatshield tiles on the panel near the rear of the shuttle.

But engineers say they have already replaced the panel, which is said to be a routine procedure.

This week's mission would be the first since the loss of Columbia and its seven astronauts in February 2003.

Discovery is due to lift off from Kennedy Space Center at 1551 EDT (2051 BST; 1951 GMT) on Wednesday.

US space agency (Nasa) engineers said they still do not know why the cover was not attached properly.

"Right now [there is] no impact to tomorrow's time and date," said shuttle vehicle manager Stephanie Stilson.

The incident is likely to be mildly embarrassing for Nasa on the eve of what it hopes will be the safest shuttle flight ever.

Pod panel

Nasa officials said the "soft" cover had fallen off by itself at about 1700 EDT (2200 BST; 2100 GMT) on Tuesday during routine work at launch pad 39B, where Discovery is being readied for launch.

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The Oms pod houses an engine and propellant for the shuttle to manoeuvre in orbit. The carrier panel seals the Oms pod to the rest of the orbiter.

"When we discovered the damage, we went ahead and pulled a spare carrier panel. The Oms pods are interchangeable, so the carrier panels are as well," said Ms Stilson.

"We found that it fitted perfectly."

The repair procedure takes about an hour to complete.

The soft window cover which fell is made of plastic with a foam edge and is taped in place to protect the windows and keep them clean before launch.

'Written in blood'

Earlier, Nasa administrator Dr Mike Griffin had given a frank press conference outlining the importance of launching the shuttle.

"There is no recovery from the mistakes we've made, whether it's going back to the Apollo fire, the loss of Challenger, or the loss of Columbia; going back to go through 100 years of flight, the lessons that we who fly know are written in other people's blood," he told reporters at Nasa's Kennedy Space Center in Florida.

"Obviously, it is utterly crucial for Nasa, for the nation, for our space programme, to fly a safe mission," Dr Griffin added.

"We have done everything that we know to do."

Its mission is scheduled to last 12 days, with landing set for 1101 EDT (1601 BST; 1501 GMT) on 25 July at the Kennedy Space Center.

The mission will carry spare parts and other equipment to the International Space Station.

Columbia and its crew of seven perished as they flew home for landing, the result of undetected damage from a debris impact during lift-off that had no effect on the ship in space.

One final obstacle to Wednesday's launch may be stormy weather forecast for the area.