Laura Michelle Kelly is used to belting out numbers while hanging 15ft above the stage. When she gave up playing Mary Poppins in the West End, she thought those days were over. No such luck. In her latest role, as Galadriel, queen of the elves, in the new musical adaptation of The Lord of the Rings, the 26-year-old leading lady spends much of her time looking down on hobbits from her wires, while singing powerfully of the delights of her Middle-earthly haven, Lothlorien. “It may look easy,” Kelly explains, “but it’s actually incredibly physically demanding. I had to spend months getting bodily fit to play this part. We all did. The stamina everybody needs to carry it off is amazing.”

“Shakespeare meets Cirque du Soleil” is what the show’s director, Matthew Warchus, calls it, appropriately. Warchus has a roomful of awards, for productions ranging from the classics to opera and new plays such as Art, and he admits that he was sceptical, at first, as to whether an adaptation of Tolkien’s beloved doorstop could work: “Not because you can’t put big novels on stage,” he says. “You can. Les Misérables is a good example. But when it comes to epics, you think how much easier it would be to spoof it. The idea of this seemed risible.”

What brought Warchus round, after he had reread the books, was the realisation that “all the things that happen in the story are things the stage can do quite well. It has taken cinema 100 years to work out how to render huge armies, battle sequences, ghosts, mountains, floods and avalanches. Theatre works in a more abstract and poetic way, and it relies on the audience’s imagination to complete the story”. Having settled his anxieties on the artistic front, Warchus worried that this project would take two whole years out of his life. “I was concerned about all the opportunities I would have to pass on.”

That was four years ago. The producer, Kevin Wallace, has been plugging away at the show for much longer. He first became interested in the possibility of translating Tolkien to the musical stage after helping a friend with a doomed version of The Hobbit, which opened, then swiftly closed, in a tent in Berlin in 1997. Wallace blamed a lumpy script and the inappropriate Broadway styling of the music for the show’s failure, but thought the basic idea felt solid. “I’d never read the books,” he says. “But I loved the story, and I thought that, done right, it could work well as a musical.”

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It was this belief that prompted Wallace to leave his post as an in-house producer for Andrew Lloyd Webber’s Really Useful Group and set up his own company in 2001. Two years later, he had Warchus in place as director and co-writer, and Christopher Nightingale on the case with the music. He also had an unforeseen competitor in the shape of the director Peter Jackson. The appearance of Jackson’s wildly popular film trilogy, from 2001 onward, robbed the stage version of its element of surprise and raised the bar in terms of what new, young Tolkien fans might expect, visually, from a stage show.

So it’s no surprise that getting the musical into its present shape has involved five rewrites and any number of technical rethinks. “There was no template or expertise we could apply to this,” Warchus says, blaming a variety of factors, from the unusually knotty plot to the production’s triple-action revolving stage, for the delays. Then, when the piece was finally ready in 2005, it turned out that no available theatre in London or on Broadway was large enough to house it. So, with the clock ticking away expensively on a payroll numbering 200, and a budget creeping up to £12.5m – making this the most expensive musical London has ever seen – Wallace took The Lord of the Rings to Toronto for its world premiere last March.

This was the point at which he and his team must have felt that the travails of Frodo and his small pals in Middle- earth were a doddle compared to fending off obituary notices such as this one from The New York Times, whose critic dismissed the musical as “largely incomprehensible ... everything ends up lost in this $25m adaptation, including plot, character and the patience of most ordinary theatregoers”. In a parallel strike, the Toronto Star headlined its review “Why we’re bored of The Rings”, while a visiting critic from Britain complained that there was “nothing here to rival the imaginative visual coups and heart-tugging emotion of such great family shows as Billy Elliot, The Lion King and Mary Poppins”.

Showing hobbit-like determination and a calm eloquence worthy of Gandalf himself, Wallace put a brave face on things when, in September last year, he abruptly closed the Toronto show after only six months. “We have made theatre history here,” he declared, but he also promised that The Lord of the Rings would reappear in London in the summer of 2007. And Wallace spake wisely. His production, almost entirely recast, with more music and 25 minutes shaved off the running time to bring it in at three hours, is now previewing at the Theatre Royal Drury Lane, and will open for critical scrutiny, after a lengthy bedding-down, on June 19.

The widely received notion that the transferring musical has already been branded a flop is an understandably sensitive – and hotly contested – point with all concerned. Wallace insists that the show sold nearly 90% of the seats at the Princess of Wales Theatre in Toronto. The chief deciding factor in closing early was the need to ship the enormous set to London before the Hudson River froze over. It took four months, and the complete removal of all the existing understage equipment, to install the Rings’ elaborate machinery in the Theatre Royal.

Warchus is cautiously optimistic that important lessons were learnt in Toronto. Market research carried out last summer has led to a simplification of the plot, “to make it more dynamic and visceral, and less demanding in terms of information,” he says. “I’ve gone as much as possible for enchantment.” Kelly, too, is upbeat. She sees this radical reworking of Tolkien’s epic as bringing something new to what has, in the wake of the blockbuster film trilogy, become dangerously familiar territory. “This is a completely different sort of musical. It does things on stage that theatre audiences have never seen before, in terms of the aerobatics, the choreography and the special effects.” Filmgoers, she is firmly convinced, will love it.

So, too, she believes, will serious music-lovers. As an accomplished and ambitious singer – with a parallel career as a solo performer, one album to her name and another in the pipeline – Kelly is completely sold on the show’s multicultural musical aspect. “The harmonies are incredible, andthe way the music expresses the different cultures of Middle-earth is really clever.” Tricky to pull together, too, apparently. The Lord of the Rings score began after a meeting in an Indian restaurant in London in 2003, set up by Nightingale, between the top Bollywood tunesmith AR Rahman and a Finnish folk ensemble, Varttina. Whether the results will sell the show to the Mary Poppins/Billy Elliot crowd remains to be seen. But they are undeniably – that word again – different.

This production will probably be remembered more for its sights than its sounds, however. The sheer scale of the beasties – the giant black furry spider; Balrog, the humungous redeyed demon from the underworld; and, tallest of all, the 20ft stilted ents – gives this Rings the feel of a Rio carnival reenacted in Covent Garden.

None of the speaking actors creates as vivid an impression as the snorting, leather-clad orcs, who power-skip and somersault across the stage like warthogs in bondage gear, and nearly steal the show when they cavort among the front rows of the stalls during the break between Acts II and III. Alongside these circus stunts, there are illusions, such as the vanishing of Bilbo Baggins in Act I, and a succession of back-projected images that hover above and behind the action. Warchus’s bid to fashion what he calls “total theatre” is about as total a spectacle as this theatregoer has witnessed.

Now there is another pause, until the critics return their verdict on June 20. As a creature of the world of subsidised art theatre, Warchus is suspicious of what he sees as the crude judgments of the mainstream commercial market. “The day after this opens, we will be disproportionately congratulated or disproportionately abused. That reception does not represent a validation of what we’ve done, I believe.”

The view from the stage, after a handful of previews, is less guarded. Kelly is convinced the show carries just as much popular clout as Mary Poppins. “It is absolutely intense, and I can see that when I watch the faces of the audience. This is a highly emotional story about tragedy, romance and how powerful friendships can conquer loss. The people who have come to see it so far have been completely lost in it.”

The Lord of the Rings is previewing at the Theatre Royal Drury Lane, WC2

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