The forward island, or missions control centre, which will include the bridge of the Queen Elizabeth carrier, has been built at the BAE Systems dockyard at Portsmouth.
The 680-tonne section has been loaded on to a barge which will depart next week for the Forth where final assembly will take place.
Paul Bowsher, QE class project leader for BAE Systems, said: "The forward island is a key part of the Queen Elizabeth aircraft carrier and once again the exceptional standard of construction we have achieved is true testament to the skills and talents of our team.
"It is a very proud moment to see her leave the hall, but we are already looking forward to her return home to Portsmouth as part of the largest warship ever constructed for the Royal Navy."
The section, known as upper block 07, will contain approximately 100 vital mission systems compartments and is the first block to depart with its final paint colours applied, all windows fitted and consoles installed, along with 43km of cables and 3101 pipes.
It has already been decorated with the HMS Queen Elizabeth crest.
Describing the carrier's specifications, a BAE spokesman said: "The ship's main bridge has floor-to-ceiling windows, which are up to two metres tall to ensure a level of visibility far beyond previous aircraft carriers.
"HMS Queen Elizabeth is the first aircraft carrier to use an innovative design of two islands.
"The second 'aft island' operates as an airport control tower to co-ordinate aircraft movements, but both islands are designed with the ability to incorporate the other's role in an emergency, thus increasing the survivability of the ship."