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It has been billed as the biggest comeback in pop history.

But thousands of Michael Jackson fans have been left disappointed even before a note has been sung.

Organisers of the star's lucrative 50-date London tour at the O2 arena, which begins in July, have cancelled the first four opening shows blaming 'technical issues'.

The dates have already been rescheduled but it now means virtually all of the 80,000 fans who had bought tickets will have to wait up to eight months to see the concerts. Many of them live abroad and will now have to make new travel arrangements, which could cost hundreds of pounds in cancelled flights.

The show's director Kenny Ortega took the decision yesterday, with a spokesman saying a clash with Madonna, who is also performing at the venue at the beginning of July, had left Jackson with no time to rehearse at the arena.

The opening show on July 8 is now being moved to July 13. But fans with tickets for three other initial concerts will have to wait around nine months until spring 2010 if they want to make it to the rescheduled concerts.

A show scheduled for July 10 will be moved to March 1 2010, a July 12 date will be moved to March 3 2010, and a concert for July 14 will now take place on March 6 2010. Randy Phillips, President and CEO, AEG Live who are organising the concerts said: 'Kenny and Michael are, at the same time, both creative pioneers and perfectionists.

'This show has grown in size and scope, thereby, necessitating more lead time for manufacture of the set, programming the content for the massive video elements, and, most importantly, more time for full production and dress rehearsals.  

'As much as we agonised over this change in the original schedule, we are sure the fans will understand when they experience the level of entertainment Michael Jackson intends to deliver while also ensuring the safety of the musicians, cast and crew and the crisp execution of the production.

'It is AEG Live’s sincere hope that those lucky enough to have purchased tickets will have enough time to change their personal schedules and travel plans if they are coming from outside of London.

We do not want anyone to miss this once-in-a-lifetime experience.' The postponements will be a huge blow to fans as Jackson is already facing questions over his fitness for the sell-out concerts after it was claimed he is fighting skin cancer.

The 50-year-old, who is currently rehearsing in the US, was reportedly diagnosed last month after doctors saw spots of cancer on his neck and pre-cancerous cells on his face, although it has been denied by his representatives.

His comeback has also been marred by planned tube closures on the London Underground in July and August. Scheduled engineering works mean the only underground station to the venue will be shut down for a number of weekend concerts.

Organisers said that fans who had bought tickets to the original shows had been notified by email and that those who could not make the new dates would be offered a full refund.

Tickets cost between £50 and £75 when they went on sale, although thousands were immediately resold through online auction sites for thousands of pounds.

Ortega added: 'After directing the Dangerous and HIStory tours, I know what the fans expect and myself and the team want to deliver a flawless production. 'We apologise to all disappointed Michael Jackson fans and remain extremely dedicated and focused on creating an exceptional live music experience."

Today, Jackson emerged from a medical centre in Beverly Hills, unusually exposing his face in a bid to dispel rumours he has skin cancer.

He is usually seen wearing surgical mask, but on this occasion he showed off his pinched nose, cleft chin and pale skin. Jackson's spokesman, Dr Tohme Tohme said: 'He is in perfect health. He doesn't have any disease.'