A ceremony is being held as the Royal Navy's new flagship and sole remaining aircraft carrier is brought back into service on Thursday.
Illustrious has been out of service for two years undergoing a refit
A £120m refit of HMS Illustrious has been completed which the navy says allows her to carry more aircraft than any previous carrier of its kind.
Her rededication comes a day after the navy's previous flagship, HMS Invincible, was mothballed.
A second carrier, HMS Ark Royal returns in six months' time after a refit.
Critics have raised concerns that her retirement leaves the navy overstretched.
The low-key event will see hymns and prayers said for the ship's crew and families on deck off the carrier's Portsmouth base.
Illustrious has been out of action for two years, as work took place to allow her to carry GR7 and GR9 Harrier jump jets.
Internal changes also provide better accommodation for the navy's newest aircraft, the Merlin anti-submarine helicopter.
A Ministry of Defence spokesman said: "The armed forces view the strike carrier as a potent power that is sustainable around the world, providing an immediate and tangible statement of intent both to Britain's allies and adversaries.
"The capability provides the Royal Navy with a versatile maritime force at the cutting edge of technology and improves the ability of the navy and RAF to provide flexible, deployable and independent strike air power."
The navy plans to replace its whole fleet of aircraft carriers in 2012.