The seven crew members aboard Discovery had been preparing for their re-entry into Earth's atmosphere.

Low-lying cloud which could obscure the landing strip forced flight controllers to scrub Monday's two landing windows at Nasa's Kennedy Space Center.

Discovery's mission is the first flight for the shuttle since Columbia broke up on re-entry in February 2003.

The shuttle was originally due to land at 0946 BST (0446 EST; 0846 GMT). The crew were told to circle the planet again in the hope that weather problems would clear.

But a persistent cloud deck at low altitude finally forced mission controllers to give up on bringing the shuttle down for 1121 BST (0621 EST; 1021 GMT).

The shuttle's first landing opportunity on Tuesday is at 1007 BST (0507 EST; 0907 GMT) at Kennedy Space Center.

Back-up strips are scheduled at Edwards Air Force Base in California and White Sands in New Mexico.

While the forecast for Kennedy is similar for tomorrow, the outlook at Edwards Air Force Base is favourable all week.

Homeward bound

The US space agency's administrator Dr Mike Griffin said there was "no agony" over the delay.

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"We're going to land one way or another, one place or another, and all we're talking about is where," said Dr Griffin.

Discovery's crew were woken on Monday at 0130 BST (2030 EST; 0030 GMT). The seven astronauts immediately began preparing for landing.

"We're getting ready here and we're looking forward to coming home," Discovery commander Eileen Collins told mission control in Houston, Texas, earlier on Monday.

When mission control finally gives its approval, Discovery will fire its engines for three minutes nine seconds at an altitude of about 340km (220 miles), to slow it down enough to begin the descent.

The shuttle will pitch up as it starts to experience the burning effects of the atmosphere at an altitude of 129km (80 miles) and a speed of about 27,360km/h (17,000mph).

After Discovery has banked from side to side in order to dissipate speed, Commander Collins will take manual control to pilot the vehicle to touchdown.

Anxious wait

Space shuttle Columbia's external tank shed a suitcase-sized piece of insulating foam during launch that punched a hole in the ship's wing, allowing super-heated gases to pierce the airframe during re-entry and pull it apart.

But onboard cameras captured multiple pieces of foam being shed from the tank - one of which was only slightly smaller than the one that doomed Columbia.

Nasa has grounded the fleet until it has made changes, so Discovery's visit to the International Space Station (ISS) may be the last shuttle mission for some time. On Saturday, Discovery undocked from the station before flying around the space lab to check it for wear and tear.

Safety concerns

Because of safety measures put in place after Columbia, Discovery has been videotaped, photographed and laser-inspected.

The astronaut removed two loose cloth strips, which were protruding from the shuttle's belly.

The US space agency ruled out another spacewalk to fix a thermal blanket, which has billowed out under a cockpit window.

There had been some concern that part of it could tear off during re-entry and hit the orbiter.