As Michael Jackson once said: Don't stop till you get enough. Concert promoters on Wednesday more than doubled Jackson's summer run of concerts in London after advance tickets sold out within hours of going on sale. AEG Live said an advance allotment of half the tickets for the 10 shows in July sold out soon after going on sale. Eighteen concerts have been added into September, and more shows are possible. Ticket-seller Ticketmaster apologized for problems experienced by fans after its computer system crashed. Managing director Chris Edmonds said the glitches were the result of "an unprecedented level of demand." Tickets, priced between Â£50 pounds and Â£75 pounds, are currently on sale to buyers who registered on-line. Half the tickets for each show were held back and went on sale to the general public Friday morning. The 50-year-old King of Pop has said the shows at the 20,000-capacity O2 Arena in south London would be his last in the British capital. The London shows are entitled "This is It," and Jackson said they would be his swan song - although AEG said the gigs could be part of a bigger, final world tour.