SpaceX's second flight to orbit has been delayed by technical problems.
The California-based company was due this morning to send its Dragon capsule into orbit on the back of a Falcon 9 rocket from Nasa's Cape Canaveral launch pad in Florida.
However, with just seconds to go, the launch was cancelled and the engine cut off.
Nasa tweeted: "The SpaceX team is backing out of their countdown and safeing the rocket. According to the team, the chamber pressure on engine #5 was high."
It's hoped that the rocket will be able to launch on Tuesday instead.
The cut-off came as a surprise as Nasa had previously posted a message that read: "Fueling is complete on the Falcon 9 rocket. The skies are clear. Things are looking good."
In December 2010 SpaceX became the first ever private company to launch and recover a spacecraft from Earth orbit.
The Dragon craft is scheduled to operate in space for about three weeks, including a rendezvous and berthing with the Space Station - a first for a private firm.
If successful, the mission will give the company another place in the record books.
This rocket and spacecraft will not carry people, but will have about 1,200 pounds of supplies onboard for the six astronauts and cosmonauts working on the Space Station.
SpaceX is working closely with Nasa on the mission, as the US space agency wants private industry to deliver cargo to the orbiting laboratory on a regular basis.
The mission will include an extensive set of tests in space requiring the Dragon spacecraft to show that it can move precisely in orbit and approach the space station carefully.
Only after these tests are successful will the spacecraft be allowed to approach the orbiting laboratory close enough to be grappled and berthed by the station's robotic arm.
A dress rehearsal for the launch took place on 30 April that concluded with a brief engine firing to verify the company's Falcon 9 rocket is ready to launch.
The Falcon 9 is powered by nine Merlin engines, and SpaceX reported that all nine were lit and run at full power for two seconds during the test.
The rocket's second stage is powered by a Merlin vacuum engine, which runs on refined kerosene and liquid oxygen, the same fuel and oxygen combination that was used on Nasa’s Saturn V moon rocket first stage.
The practice countdown also tested some of the systems on the Dragon spacecraft that will fly to the space station.
"Woohoo, rocket hold down firing completed and all looks good!" reported Elon Musk on his Twitter account.
Musk is the owner and chief designer for SpaceX, or Space Exploration Technologies.