LONDON (Reuters) - Elton John was last at Westminster Abbey 14 years ago when he sang the haunting "Candle in the Wind" at the funeral of his friend Princess Diana.
On Friday, the British singer will be back as a guest at the wedding of her elder son Prince William, a happier occasion at which his mother's absence will nonetheless be keenly felt.
More than one million people lined the route of Diana's cortege in 1997 to pay their respects to a woman who was a royal outcast by the time of her death aged 36 in a Paris car crash but a hugely popular figure with the public.
For many, the abiding memory of William is still as a 15-year-old boy, head bowed, following his mother's coffin through the packed streets of London watched by hundreds of millions more people around the world.
"The last time we were in Westminster Abbey, my heart sank when I saw those two boys walking behind the coffin," Elton said, referring to William and his younger brother Harry.
"I can't imagine at that young age having to walk in public following your mother's coffin," he added in an interview with U.S. television host Barbara Walters.
"And the next time we are in the Abbey it's to see him walking up the aisle with a beautiful woman and the love of his life," said Elton, who was knighted by Queen Elizabeth for services to music and charity, adding:
"I think it's the most joyous result and I'm sure Diana would be very, very happy about it."
Media executives say the "Diana connection" is one reason why the public appetite for coverage of the wedding, particularly in the United States where she had an army of admirers, is so strong.
Elton John's "Candle in the Wind 1997" became the best-selling single worldwide since charts began with 33 million copies sold.
Royal observers say William has made a point of keeping his mother's memory alive as Britain gears up for the biggest royal occasion since the Queen Mother's death in 2002.
Last year, William, 28, presented his 29-year-old fiancee with his mother's dazzling sapphire engagement ring and the couple reportedly visited Diana's grave at Althorp Estate, home of her family in central England, ahead of the big day.
"I'm sure the royals would like everyone to forget Diana ever existed, but this young man is never going to allow that to happen," said Joan Lunden, a U.S. TV host who covered Diana's wedding in 1981 and is part of Fox News' royal wedding team.
"Elton John was an obvious move and the ring, and who knows what else we'll see on Friday that will connect the two.
"While William is concerned Kate is going to become the next Diana, it's another fairy tale -- she's the beautiful princess and now we want the happy ending."
An international media village has sprung up to provide coverage of the royal wedding and a British minister has said the global television audience could reach two billion people.
"Princess Diana was the biggest celebrity in the world and her son's wedding is an extension of that," said Linda Bell Blue, executive producer for entertainment news show "Entertainment Tonight" and "The Insider."
"We don't have royalty in America, we are an extension of Britain by our roots," she told Reuters. "This is a glimpse into Diana really that Americans have been missing."
Diana's ill-starred marriage to Prince Charles and the nature of her death have inevitably prompted words of warning about Middleton's future in the intense media glare.
A survey of media coverage has already suggested she could eclipse even Diana in terms of fame and celebrity.
"The only downside (of the wedding) is the sad side," Lunden said. "That thought of 'Gee, I wish Diana was here.'
"You can't help but have the thought that the second she walks out of Westminster Abbey, Kate is going to become the next Princess Diana and I think it's inevitable it's going to happen.
"Speculation about the wedding dress will instantly be replaced by speculation about the first baby."
Reporting by Mike Collett-White, editing by Peter Millership