Make it a double! That was Southampton's order of the day when the city's identical twin superliners, Adonia and Oceana, were christened in a unique royal celebration last night.

The Queen's glamorous granddaughter, Zara Phillips and her mother, the Princess Royal, sent the champagne fizzing and fireworks flying when they named the lookalike cruise ships, dubbed Southampton's White Sisters, in a dazzling dockside ceremony, the biggest of its kind in Europe.

Rockets soared into the sky sending showers of sparkling colour cascading over the ships as Adonia's and Oceana's powerful whistles boomed out across the docks in a salute to their new home.

Everyone was seeing double during the event, the first of its kind ever staged in Great Britain, in which Zara and the Princess Royal officially became the ships' two godmothers.

After a brief service of blessing it was the Princess Royal who began the naming by saying the emotive words: "May God bless her and all who sail in her," to christen Oceana.

Then Zara, undertaking her first ever formal public engagement, repeated the famous phrase for Adonia before the godmothers simultaneously pressed a button to release two traditional bottles of champagne against the bows of the ship.

However, in a bizarre twist which echoes the previous occasion when the Princess Royal named the P&O Cruises' vessel Aurora, in Southampton, the champagne bottle again did not smash to pieces on the ship.

Many in the audience were convinced the bottle remained intact but Captain Steve Burgoine, master of Oceana said it had cracked allowing the bubbly to leak out.

"This time the bottle did fracture although it didn't break up but at least the champagne came out," said Capt Burgoine who was also in command of Aurora when she was named.

It is an old sea-going superstition that if the bottle does not break then it is a sign of bad luck.

Nicknamed the White Sisters because of their matching appearance, Adonia and Oceana were the dramatic backdrop for the ceremony watched by 1,800 specially invited guests.

Berthed bow to bow in a mirror image of each other the massive white ships were illuminated by a spectacular light show as the quayside party began.

In the audience were ambassadors, sporting and showbusiness personalities, international shipping magnates, captains of industry together with both national and local politicians.

Names on the guest list included Saints own Gordon Strachan and Matthew Le Tissier, swimmer and TV presenter Sharon Davies, singer and actress Claire Sweeney, Olympic ice skater Jayne Torvill and entertainer Danny La Rue along with Countdown's Richard Whiteley and former Archbishop of Canterbury's special envoy Terry Waite.

For more than an hour the dockside was transformed into a stage with a mixture of music, dancing and even two skaters performing on a temporary ice rink, all starring in a show themed around double acts.

At one point to reflect the Princess Royal and Zara's love of equestrian events there was even a display of dressage by two perfectly paired white horses.

The show, compered by world championship rowing partnership Matthew Pinsent and James Cracknell, culminated in a finale featuring the band of the Royal Marines, Portsmouth, Southampton University Gospel choir and the fireworks.

From the outside the ships may look identical but step on board and straight away the differences are apparent, with each White Sister having a style, atmosphere and personality all of her own.

Oceana has all the extensive facilities of a modern ship and manages to combine glamorous surroundings with a wide range of entertainment that appeals to families.

Adonia, on the other hand, is dedicated to adults only and, with its laid-back luxury, is the most contemporary ship in the company's Southampton fleet.

Both vessels boast top class restaurants and a range of passenger facilities including sumptuous public rooms, soaring, four-deck high atriums with glass lifts that take passengers to different levels of the ship, health and beauty spas, show lounges, a theatre equipped to stage West End-style productions and many different bars.

Each has a library and casino, there are swimming pools, a promenade deck where passengers can literally take a turn around the ship, while corridors and stairways feature a major collection of original artwork.

At 77,499 tons each the ships are the largest vessels that P&O Cruises have regularly operated from Southampton and their distinctive, buff-coloured funnels are so large that they each contain a sports deck as well as housing the essential exhaust pipes from the many on-board systems and machinery.

The decision to bring Oceana and Adonia to Southampton to sail alongside the company's other two, well-established ships Oriana and Aurora comes in response to the continuing growth and substantial future prospects for the multi-million-pound UK cruise industry which has its main centre in the city's port.

The addition of the two ships gives P&O Cruises the most modern fleet of superliners, flying the Red Ensign and staffed by British officers, completely dedicated to British passengers.

Both ships have undergone a limited facelift to change areas that were initially designed for American passengers to appeal more to British tastes.

One of the Princess Royal's favourite charities, Save the Children, benefited from the naming ceremony and the gala night on board Adonia and Oceana.

P&O Cruises donated tickets for the event to the charity to make cash for the organisation's work and fun fundraising casinos were held on both ships during the evening.