PICTURE the happy scene. It's the blushing bride's big day. She walks down the aisle to the waiting groom while in the congregation proud parents beam as the couple tie the knot.

However, these days it is sometimes a seamen's knot that binds the bride and groom together as more and more couples opt for a wedding at sea and this is big business for many Southampton based cruise ships.

So far this year almost 200 couples have been married on the city's ships and this number is set to keep on rising.

In fact, there are no wedding wishes, such as fresh flowers, photographs, invitations and even a specially made three-tiered wedding cake, that companies such as P&O Cruises cannot fulfil.

The enduringly romantic notion of being married by a captain at sea can now be a reality as shipping lines rush to provide a unique backdrop for a wedding.

Whether it is just a couple looking for a quiet service or a lavish affair with dozens of guests, flowers, bottles of bubbly and everyone joining the honeymoon cruise there is a ship that will arrange it.

There was a flurry of confetti earlier this week on board Southampton's Oriana when P&O Cruises announced the vessel was giving up British registry and switching to Bermuda - all because the company wanted to be able to hold weddings at sea.

It is a long-held misconception that a couple can run away to sea and be married by a ship's captain.

In reality it's far more complicated. Under Britain's Marriage Act of 1949, weddings can only be held in a "permanent and identifiable place'' and a vessel ploughing through the Atlantic does not meet the lawful requirements.

So when couples began increasingly to ask if they could be married at sea, shipping bosses - quick to seize the chance to boost passenger numbers - looked around to see how they could make maritime marriages legal.

The answer lay 3,460 miles away in Bermuda, already a popular registry, not least because the islands in the North Atlantic, unlike the UK, invested the power to conduct legal weddings with the ship's master.

When Oriana arrives in Southampton on December 18 after a refit, she will have completed her registration with Bermuda and the crew will be able to welcome brides and grooms.

Oriana will join other P&O Cruises' ships in Southampton such as Arcadia, Artemis and Oceana already registered in Bermuda.

The company has made no decision on whether Aurora, will follow.

Seamen's trade unions worried about cruise ships leaving the British registry so they become venues for weddings have for some years lobbied the government to change the rules of the Marriage Act.

Andrew Lingington, a spokesman for Nautilus UK, formerly NUMAST, said: "Marriage services make the cruise companies a lot of money and we have tried to persuade the government to change the rules to allow weddings to take place on UK vessels.

"The trouble is we keep asking the government but it still has not found the parliamentary time to make the changes.'' It is understood that there is no likelihood in the foreseeable future that weddings will be permitted on UK registered ships at sea and so as demand for ceremonies of this kind increase it is likely more British ships will be forced into changing registry.