By Kevin Peachey BBC News, personal finance and consumer affairs reporter

The Ventura will be launched by Dame Helen Mirren

When Alan Wase spent �1,500 on a ticket for superliner Ventura's maiden voyage, he thought he would be the first to set sail on her.

The 63-year-old cruise veteran had no hesitation in signing up for the trip to Barcelona, Florence and Gibraltar with his wife and four friends.

Ventura, operated by P&O Cruises, is the largest liner yet to have been built specifically for the UK market and will carry nearly 3,600 passengers and 1,200 crew.

Holidaymakers on the 14-night cruise to four countries, which leaves Southampton on 18 April, will be able to make the most of the spa, fine dining and even a giant Scalextric track onboard.

But the cruise ship, having been built ahead of schedule, will now test the waters the week before.

It is like buying a first night ticket to the theatre only to find out later that they've started selling tickets to the dress rehearsal Holidaymaker Alan Wase

Last week, P&O decided to add an extra "welcome party" cruise which heads out on 11 April. The two-night trip from Southampton to Zeebrugge and back will take place prior to the "maiden" voyage and will cost �359.

No longer the first

Now Mr Wase, a veteran of a dozen cruises, feels he has been stripped of the opportunity to be on the first voyage.

"It is like buying a first night ticket to the theatre only to find out later that they've started selling tickets to the dress rehearsal," said the retired crime officer from Twyford, near Banbury in Oxfordshire.

"I feel they are depriving us of the uniqueness of being the first paying passengers to sail and be onboard.

"I was absolutely devastated to find that another cruise had been slipped in."

Still maiden?

A P&O Cruises spokeswoman said the earlier trip would not ruin the maiden Mediterranean voyage for holidaymakers.

"Due to an early shipyard handover and longer than expected period in Southampton, P&O Cruises has added a two-night 'Welcome Party' cruise to the inaugural celebrations for Ventura," she said.

"As it takes place before the official naming ceremony, this cruise will not in any way detract from the maiden cruise that sets sail on 18 April."

"The maiden voyage of a ship is very special and we will be making this a truly memorable holiday for our passengers, with enhanced entertainment and special events - including a spectacular firework sail-away party."

In the absence of royalty, Dame Helen Mirren - who played the leading role in the hit film 'The Queen' - will name the 115,000 tonne superliner on 16 April.

Passenger rights

Despite the disappointment of Mr and Mrs Wase and their friends, experts say that it is unlikely that they would have any kind of civil claim against P&O Cruises.

Bryan Lewin, chairman of the Trading Standards Institute, said: "I can see why the customers are upset.

"Yet, it would be difficult for people to show some real consumer detriment."