Just like her mother?

Masks to hide who's behaving badly. Limitless drinks. Dresses with eye-popping décolletage. Sound familiar? No, it's not one of Fergie's infamous thrashes, but Princess Bea's coming-of-age party...

With a smattering of celebrity and some truly fabulous fancy dress, it promises to be the most spectacular royal party since Prince William turned Windsor Castle into an African wilderness for his 21st birthday celebrations three years ago.

For Princess Beatrice, however, her 18th birthday ball at the Queen's favourite residence next Saturday marks something more than her coming of age: it represents nothing less than the return of her mother, Fergie, to the royal fold.

The gold-embossed invitation that recently fell through the letterboxes of almost 500 friends and relatives states boldly that the party is hosted by 'HRH The Duke of York and the Duchess of York'.

To most of us, it would seem straightforward that Beatrice's parents should host the party. But nothing in royal circles is quite that simple.

For more than a decade, the Palace has steadfastly refused to acknowledge the Duchess. The Duke of Edinburgh has not uttered a word to his former daughter-in-law since her divorce from Prince Andrew ten years ago.

It is inconceivable the Royal Family would allow Windsor Castle to be used as the venue for any party hosted by Fergie unless it had been agreed that a rapprochement should now take place. And the truth is that much of the credit for this must go to Beatrice herself.

With a maturity beyond her years, the teenager has been lobbying her grandparents to forgive and forget the tribulations of the past - the Duchess's toe-sucking incident in France, her once embarrassingly large overdraft, her galumphing and undignified jollity in the early years of her marriage. And by and large, these pleas have been heard.

It has helped, of course, that the Duchess has reformed her wild ways in recent years. But there is also another compelling reason for the rapprochement. The Queen is concerned to see that her grandchildren - given the difficulties of their upbringing in fractured families and their life in the royal goldfish bowl - have as much support as she can provide to prevent them going off the rails.

So with the family discord put firmly behind them, mother and daughter have set about organising the younger generation's social event of the summer, jubilant at the thought that, at long last, this will truly be a united family celebration.

For a party theme, serious-minded Beatrice has decided to explore her passion for history, plumping for an '1888 masked ball' to mark the fact that she was born a century later on August8, 1988. The theme also offers a link with her great-great-great-great grandmother Queen Victoria, after whose ninth and youngest child she was named.

It is telling that Beatrice and her parents had originally scheduled a modest dinner-dance for a select few at Prince Andrew's Windsor Great Park home, Royal Lodge.

But when the Queen, who will make an appearance on the night, got wind of the preparations earlier this year, she insisted that the entire family should get behind the occasion - and moved the soiree across to the castle itself, offering the use of her household staff.

Smoking, cameras and mobile phones, the guests have been warned, are strictly banned

There have been teething difficulties, it must be said. To save money, Andrew unceremoniously dumped leading society event organiser Sally Ann Whetherly, who had been asked to mastermind the party by her great friend, the Duchess.

But despite this, the celebration of the milestone birthday of the Queen's favourite granddaughter will be a far from restrained affair. The Duchess has called it Beatrice's 'coming out ball'.

Nearly half the guests will sit down to a Victorian feast around the state banquet table in Windsor's historic Waterloo Chamber shortly after 7.30pm.

The remainder - including many of the Duchess and Andrew's friends, as well as Beatrice's friends from St George's School in Ascot - will arrive at 10pm for champagne and canapes created by upper- crust caterers By Word Of Mouth, before dancing until the small hours. Smoking, cameras and mobile phones, the guests have been warned, are strictly banned.

Princes William and Harry will attend - Harry accompanied by his girlfriend Chelsy Davy, who will be introduced to the Queen for the first time. Lady Sarah Chatto, Princess Margaret's daughter, will also join the partygoers.

Former royal wild child Marina Ogilvy, daughter of the Queen's cousin Princess Alexandra, will be escorted by her daughter Zenouska, one of Beatrice's best friends.

Several of the Duchess's loyal supporters have been invited including ex-flame Paddy McNally - with whom Beatrice and her sister Eugenie, 16, have often holidayed - the Marchioness of Milford Haven, Julia Dodd-Noble, and society jeweller Theo Fennell and his wife Louise.

Supermodel Elle Macpherson, Formula One boss Bernie Ecclestone, the Osbourne clan and the precocious Peaches Geldof - with whom the rather straightlaced Beatrice has, surprisingly, formed a close friendship - will provide showbusiness glamour.

All have been told to go to town on their outfits. Uniform or full dress coat and trousers are required for gentlemen; one schoolboy was seen last week lovingly admiring a rare eggshell blue silk dress jacket and silver-topped cane in a London antique dealers.

Ladies have been advised to wear full Victorian evening gowns, but without plumes or trains. Guests have even been given 'administrative notes' recommending suppliers of Victoriana, including an outlet where tailored Victorian-style dresses start at £700, and London's famous Grays Antique Market for genuine antique jewellery.

According to one source, many guests are going for a 'Lillie Langtry-look' with acres of daring decolletage. Another said the wasp-waisted outfits are posing problems for some of Beatrice's friends.

A very public step into adulthood

"Let's just say they are a little too well-fed - we are being faced with a stream of schoolgirls trying to squeeze into dresses made for women who were rather smaller."

Beatrice, Eugenie and their mother are going further afield, ordering their dresses from leading New York designer Marchesa, which is owned by the Duchess's friend Georgina Chapman.

Although careful to keep the design of Beatrice's dress a secret, Miss Chapman revealed to the Mail yesterday that she had created a 'show-stopper' gown from rich, tactile fabrics including brocade and jacquard in deep blue and gold hues with a 'curve-enhancing' bustle and daring corset finished with intricate embroidery.

For Beatrice it is a very public step into adulthood, but friends say she is more than ready to meet the challenge - she has a mature head on her young shoulders.

"Remember her parents split when she was just eight and she has had to cope with the way her mother was ostracised by the public and the Royal Family," says one family friend. "With that amount of emotional baggage, she could so easily have become one of those vacuous Sloanes dabbling in drink and drugs. It is a testament to her strength of character that this couldn't be further from the truth."

Such fortitude has stood her in good stead in her battle with dyslexia, which she revealed for the first time last year.

Indeed, her parents couldn't hide their delight when she passed nine GCSEs last summer and was later made head girl at St George's, where this week she had to deal with the fall- out from the news that a fellow Lower Sixth pupil with whom she was friendly had tested positive for drugs.

Currently studying for A-levels, the princess wants to go to university in the U.S. next year and hopes to work for a children's charity before taking on full-time royal duties.

Beatrice, who is fifth in line to the throne, is honest enough to admit she enjoys the attention that comes with being the Queen's granddaughter. "I wouldn't trade my life for anyone else's," she said during her first glossy magazine interview last year. "I know I'm very lucky. I love being who I am."

Recently, she has taken to joining her mother on jet- set trips to New York and the South of France (not to mention attending the Beckhams' pre-World Cup party in an eye-popping designer dress), where she has been socialising with the notorious Euro-trash crowd of young socialites.

While family friends are pleased to see that the demure youngster is coming out of her shell, there are those who worry she should be so much under the influence of her mother, who once joked that she 'goes on the pull' with her daughter.

"Sarah and Bea get on so well that at times it is like watching two sisters together," said one friend. "But at times you wonder whether it is wise for Sarah to be treating her daughter like a 'mini-me'."

And there have been question marks - just as there were with her mother - over Beatrice's choice of men, or to be more specific, one particular man.

Her first serious boyfriend, Italian-American Paolo Liuzzo, turned out to have faced a lengthy prison sentence after being charged with the manslaughter of a student in 2002.

The 24-year-old was later given community service after pleading guilty to a lesser charge of assault and battery, and Beatrice, though shocked, vowed to stand by her beau - whom she dated for several months - before being forced to end the relationship in February.

Liuzzo has not been invited to next week's party, but we can reveal that the two are still in touch by text message and e-mail and Beatrice has promised to visit him in New York this summer.

A rare flash, then, of teenage rebellion - but one unlikely to worry the Queen who has, according to well-placed Palace sources, 'high hopes' for the princess.

"Beatrice is very conscious of her position as the Queen's granddaughter and, while she is no prude, would never let herself be seen staggering out of a nightclub in the early hours like some other young royals," said one courtier.

There will be high jinks at Beatrice's party but it's unlikely she'll be involved in any scandal.