"We have had to stop the launch countdown 42 seconds before scheduled lift-off," Arianespace President Jean-Yves Le Gall said from the company's launch center in Kourou, French Guiana on the northeast coast of South America.
"We detected an anomaly in the pressurization system of launcher so we must conduct investigations in the upcoming hours and we will not launch tonight," Le Gall said.
He said a new launch date would be announced "as soon as possible".
A first attempt to launch the rocket was halted hours before scheduled lift-off on March 26 due to a problem in a launcher subsystem.
The Ariane-5 rocket had been scheduled to launch the ASTRA 3B satellite for Luxembourg-based SES-SA and COMSATBw-2 for Germany's Ministry of Defense.