SpaceX's landmark launch to the International Space Station has been postponed due to poor weather with around 20 minutes to go until takeoff.
"Unfortunately, we are not going to launch today," SpaceX launch director Mike Taylor told NASA astronauts Bob Behnken and Doug Hurley.
The next launch window is on Saturday.
Billionaire entrepreneur Elon Musk's private rocket company SpaceX is set to launch the first spaceflight of NASA astronauts from US soil in nine years.
A SpaceX official cited a 60 per cent chance that thick clouds over eastern Florida could force a launch postponement on Wednesday.
The next launch window would be on Saturday afternoon.
There was off-and-on rain around Kennedy Space Center and the National Weather Service issued a tornado warning for the area.
A SpaceX Falcon 9 rocket was due to lift off at 4.33pm local time (6.33am AEST), launching astronauts Doug Hurley and Bob Behnken on a 19-hour ride aboard the company's newly designed Crew Dragon capsule to the International Space Station.
US President Donald Trump flew aboard Air Force One and arrived in Florida to observe the launch.
"We are go for launch!" NASA administrator Jim Bridenstine wrote on Twitter earlier in the day.
SpaceX and NASA "will continue monitoring liftoff and downrange weather as we step into the countdown", Bridenstine added.
The two astronauts made a series of preparations for the planned launch.
Mr Hurley even posted a picture of his breakfast of steak and eggs before suiting up in SpaceX's white flight suits at the Kennedy Space Center's operations and checkout building.
Mr Musk and Mr Bridenstine, wearing protective face masks due to the coronavirus pandemic, were present at the building to join the two astronauts, who did not wear face masks.
The astronauts then emerged from the building, waved to family members and onlookers, including Vice President Mike Pence, and hopped into a Tesla vehicle to drive to the launch pad.
After giving thumbs-up signs at the launch pad, the duo made their way up an 80m tower to the crew access arm, a bridge that leads to the capsule atop the Falcon 9 rocket.
They were then strapped into their Crew Dragon seats before the hatch was closed ahead of the planned take-off.
"This is a dream come true for me and everyone at SpaceX," Mr Musk said on a NASA video feed ahead of the launch.
"When starting at SpaceX in 2002, I really did not think this day would occur."
