The unmanned aircraft stores power from the sun in its batteries, allowing it to continue through the night and, potentially, stay in the air for months at a time.
Unlike conventional spy planes, which need regular refuelling and so make relatively short flights, it will give commanders an unbroken picture of the battlefield.
The engineers who created the Zephyr also believe it will have important civilian applications, including search-and-rescue operations and police surveillance.
It has been developed by the British defence company Qinetiq, which will announce the record-breaking flight tomorrow.
The plane is designed to provide surveillance and communication support for military personnel on the ground by circling slowly over a battle zone, tracking troop movements with its on-board camera. It can reach 70mph at 60,000ft.
Unmanned aerial vehicles, or UAVs, have been used heavily in Afghanistan and Iraq by American and British troops. Pilots operate them by remote control, but the Zephyr flies automatically once airborne, receiving commands via satellite.
Qinetiq aims to provide fully operational systems for the Ministry of Defence and the US Department of Defence within two years.
