Andrew Lloyd Webber is concerned it will be "quite a long search" to find the right children for his UK production of School of Rock.
The composer's Broadway production, an adaptation of the 2003 film starring Jack Black, opened at the beginning of December at the Winter Garden Theatre, where it broke the house record for Christmas week with takings of 1.5 million US dollars (£1 million) - beating previous record-holder Mamma Mia.
Lloyd Webber announced plans for a West End transfer early in December.
He told the Press Association: "I can't absolutely announce an opening date in London until I begin to know that we can find kids who can do what the American kids can do.
"I'm sure we will, but you never know how long it's going to take, and one of the things in Britain is that you have to find three casts.
"You can't work with one cast of children.
"In America you can work with one cast, and obviously the usual understudies, but in Britain you can't.
"So we have three times the work ... I would think it's going to be quite a long search.
"And we saw 22,000 children in America before we got to this cast."
Because of the UK's performance regulations on child actors, which limit the number of hours children are legally allowed to work for, the show's producers will need to find three actors to cast for each of the 13 child roles.
The composer, 67, set up the Andrew Lloyd Webber Foundation in 1992, which he hopes will help when the search starts in the new year.
He said: "We have over 3,000 children who are involved with music through my foundation.
"And we have quite a few leads, therefore, about where we could go.
"So it'll be, hopefully rewarding, because in the end School of Rock is really about the empowering force of music and what it can do for kids. That's what it really is about."
This is Lloyd Webber's first musical to translate from Broadway to the West End - instead of the reverse.
He said: "Obviously it's a thrilling outcome. To have a hit on Broadway is what one always dreams of."
This was the only way to do the show authentically, he explained.
"Because it's an American story with American actors, and if you're going to create a work that's set in America, and has an American cast, you have to work with American actors first," he said.
"If you worked with British actors you wouldn't get the real thing.
"It's much easier now for British children to - because it's had a blueprint in America - it's much easier for them to play the role having seen the ones played by Americans."
Lloyd Webber also stressed how "tough" it was to develop a musical from scratch.
He said: "In America it's horrendously expensive to produce a show there.
"School of Rock cost 16 million US dollars (£10.8 million) to produce and a musical of that scale in London today is going to cost a minimum of £5 million.
"So it's very, particularly if you're developing one from the beginning, they are expensive things.
"And they're very, very risky."
Explaining the show's success so far, he said: "It's a great story, and as I said earlier it's about how music can empower children and change people's lives, and that's exactly what - I think it tells that story very simply.
"The original movie did, and we take it a bit further.
"And it's just a very simple tale.
"It is entirely in the end about how music can fulfil and how it can change your life."