THE London-based P & O Shipping Line has ordered its largest and most innovative cruise ship. The 77,000 tonne super-liner, to be named Sun Princess, will be delivered in December 1995 and join the line's worldwide cruise fleet of 14 ships operating in North America, Europe and the western Pacific. The 1,950 berth vessel, costing US$300 million, will be assigned to Princess Cruises, P & O's Los Angeles-based subsidiary, which is a leader in the North America premium cruise market. Key features of the ship will be its high level of technical specification and spaciousness, continuing the tradition of the most recent Princess ships, Crown Princes and Regal Princess, which were delivered in 1990 and 1991 respectively. Sun Princess will share the designer used to create P & O's revolutionary Royal Princess, which set new standards of style and elegance when introduced in 1984. Again, the design of the newcomer is said to be ''extremely innovative'' in giving passengers wide options for dining and leisure activities. At least 70 per cent of the outside cabins will have private balconies, which - more than any other ship - plus five different dining areas with two main dining rooms. A luxurious health centre and spa surround a pool suspended between two decks while two atrium lobbies are surrounded by shops, lounges, speciality bars and a casino. A spectacular cobblestone, wrought-iron pizzeria is another feature, together with a conference centre that will accommodate up to 300 people. Chairman of P & O Cruises Tim Harris said: ''This ship's technical specifications and cabin and public area design and facilities set a new industry standard that will take us well into the 21st Century.''