Michael Jackson, shown earlier in March announcing his This Is It shows in London, is adding concert dates because of strong ticket sales. (Joel Ryan/Associated Press)

Promoters in Britain have more than doubled Michael Jackson's summer concerts in London after advance tickets sold out within hours of release, crashing the ticket system.

AEG Live announced Wednesday that 15 more This Is It concerts are being added to Jackson's 10 July shows, taking the run into at least September.

The promoter said advance tickets for July went swiftly, hitting a snag, soon after Jackson made his announcement March 5 in London.

Ticketmaster's managing director, Chris Edmonds, said the crash was the result of "an unprecedented level of demand."

The 50-year-old performer said his London concerts at the 20,000-capacity O2 Arena would be his swan song, but AEG officials said the gigs could be part of a final world tour.

Half the tickets, priced between £50 and £75 ($89 and $133) for each show, have been held back and go on sale to the general public Friday morning.

The London concerts are highly anticipated, in light of the King of Pop's withdrawal from public life since being acquitted of child molestation charges in California in 2005.

There are also doubts about his ability to sing for an extended time.

His last live performance in Britain was at the 2006 World Music Awards, when he whispered a few lines of We Are the World before leaving the stage.

The singer has also struggled to pay debts and was was forced to give up the deed to Neverland, his ranch and miniature amusement park in California, last year.

Jackson has sold more 750 million albums and won 13 Grammys, but he has not staged a major tour since 1997 or released an album of new material since 2001.

With files from the Associated Press