Michael Jackson announced his This Is It concert series in March, saying the shows would be his final performances in London. (Joel Ryan/Associated Press)

Michael Jackson's sold-out, 50-concert "final curtain call" in London could be in jeopardy amid a new threat of legal action against the troubled performer.

New Jersey-based music promotion company AllGood Entertainment Inc. is seeking to block Jackson's upcoming concert run — slated to begin in July and run through February 2010.

AllGood Entertainment officials allege they signed a deal with Jackson's manager, Frank DeLeo, for a Jackson 5 reunion in July 2010, which is also to include sister Janet.

Under that agreement, AllGood Entertainment alleges, Jackson agreed not to give another concert until after that date.

In March, British music promoter AEG Live announced what it called an unprecedented series of concerts billed This Is It — suggesting a swan song for Jackson, whose troubled personal life now often overshadows his musical achievements.

The initial 10-concert run at the 20,000-capacity O2 Arena in London immediately sold out. AEG eventually boosted the offering to 50 shows. The entire series sold out within a few hours.

Jackson has largely withdrawn from public life after being acquitted of child molestation charges in California in 2005. Still, the multiple Grammy Award-winner and bestselling singer continues to make headlines for his erratic behaviour, rumoured ill health, ongoing financial struggles and a regular slate of lawsuits brought against him by former associates.