The vessel will embark for Turkey and the same shipyard in Leyal that stripped down her sister ship, HMS Invincible. The deal is worth £2.9m.
Crowds - including former crew members - were expected to gather in Portsmouth to see her off at around 1pm.
Ark Royal saw active service based in the Adriatic during the Balkans War and in the Gulf for the 2003 invasion of Iraq.
She was decommissioned as a result of the Strategic Defence and Security Review in 2010.
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Rust in Peace oh metal one