The Portsmouth-based aircraft carrier has gone through a £12m upgrade and will set off for two weeks of post-maintenance tests.

She is due to pass the Round Tower in Old Portsmouth about 10.30am, to head for the navy's training areas off the south west of England.

During the upgrade her hull was painted with new 'intersleek' paint - allowing her to cut through the seas more quickly - which will cut fuel costs and emissions by nine per cent.

It will also increase her top speed by two knots to 30 knots.

The work, carried out by BVT Surface Fleet, included new exhaust systems, an updated computer network and improvements to sewage treatment plants.

Her gearboxes, main engines and diesel generators have also been refurbished.

Ark Royal will resume her role as Fleet Flagship in the New Year, taking over from her sister ship HMS Illustrious.

Captain John Clink, in charge of Ark Royal, said: 'The ship's company is excited and proud to be taking Ark Royal back to sea to start our preparations to become the Strike Carrier and Fleet Flagship.

'I recognise that our seven months in Portsmouth has been hard work but it has ensured that we are ready to start our sea training and be able to practice our war-fighting skills.'