Right now it is a massive revolving wheel, a complex web of wires, struts and machinery.

But in a few weeks it will be transformed into Middle Earth for what is easily the most expensive musical to be seen in the West End.

This moving floor - nearly £1million worth of automated engineering - will be the basis of the set that will see the stage of the Theatre Royal Drury Lane turned into mountains, valleys and battle scenes for the £25million production of The Lord Of The Rings.

With three "revolves" - as the rotating wheels are known - and 17 "lifts", all of which can move independently, it will allow the show's producers to change the shape of the set to an almost-unprecedented degree without the actors having to leave the stage.

Previously, the most expensive West End production was Chitty Chitty Bang Bang, which cost £6.2 million to stage.

Lord Of The Rings producer Kevin Wallace said: "It's like a landscape changing as the actors move through it and over it. It's like you're watching something which is almost cinematic visually - but it's three-dimensional."

One of the most dramatic scenes where the revolving stage will be used to full effect is the battle of Helm's Deep. It will rise and fall to show the Orcs first preparing for battle and then storming the citadel. Another is in the first act, when Frodo and the other hobbits are fleeing the Black Riders.

Wallace said: "It subtly changes its configuration as they are running through the Ford and along the roads of Middle Earth, trying to get to Rivendell. It is constantly changing."

The stage, built by Delstar Engineering of Suffolk, cost £450,000 to construct and about the same again to automate, he added. The production, which will start previewing in May, has been changed substantially since it was first seen in Toronto last year, where it was greeted with mixed reviews.

It has been cut down to three hours and will feature new musical numbers by the show's composer AR Rahman.

Wallace said: "What we have made is a piece of musical theatre for non-Tolkien audiences."

James Loye, from the Canadian production, plays Frodo, Malcolm Storry is Gandalf and Laura Michelle Kelly is Galadriel.