The launch of the UK's Skynet 5B military communications satellite from French Guiana has been delayed.
The spacecraft's Ariane rocket has developed an electronic problem in one of its solid fuel boosters and will be rolled back to an inspection shed.
The flight, originally timed for Friday night, is not now expected to take place until Monday at the earliest.
The ?3.6bn Skynet project is designed to give British commanders access to more information, much faster.
"We tried to understand what's going on [in the electronic equipment] but unfortunately we did not succeed," explained Jean-Yves Le Gall from Arianespace, the company which runs the Kourou spaceport in French Guiana.
"We are now taking the launch vehicle back to the Final Assembly Building where we will investigate the equipment and replace it."
The countdown was stopped six hours, two minutes and 34 seconds before the scheduled lift-off.
The Skynet 5B platform is set to join in orbit the 5A satellite, which was lofted successfully in March and is already handling secure traffic for UK forces in Iraq and Afghanistan.
The spacecraft, which have been developed by EADS Astrium in Stevenage and Portsmouth, provide-two-and-a-half times the bandwidth capacity of their predecessors, the Skynet 4 satellites.
The greater performance is necessary because military commanders are starting to use information-rich applications, such as video, in their operations.
Only today, the Ministry of Defence announced it had started flying its new Reaper Unmanned Aerial Vehicles (UAVs) in Afghanistan. The planes will gather intelligence on Taliban activities and their pictures can be fed back to the UK via Skynet 5A for further analysis, if necessary.
The problem with the Ariane rocket also delays the launch of Skynet 5B's co-passenger - a Brazilian satellite called Star One C1 which will deliver broadband internet services to consumers in South America.
Delays to launches are not uncommon. Officials constantly monitor the rocket and its payload and if there is the slightest doubt, the flight is stood down. Skynet 5A was itself held on the ground for 24 hours when a fault developed in the launch table.
"This is the price for total quality; we do not want to take any risks, and this is why we replace equipment anytime we could have a problem," said Mr Le Gall.
The technical teams responsible for the satellites will continue to monitor their systems over the weekend. Umbilical connections to the Ariane allow engineers to talk to the spacecraft even though they are closed inside the top of the rocket.
"At this stage, the thing we worry about is the battery [in 5B]," explained Patrick Wood, the Skynet programme chief at Astrium.
"The team will be monitoring the charge on the lithium ion battery, and checking it is on external power. Once we've got a resumed launch time, we will go with our standard test sequence," he told BBC News.