The future of a plan to sell unused “air rights” above the Los Angeles Convention Center was uncertain Tuesday following a rare veto by Los Angeles Mayor Antonio Villaraigosa.

Villaraigosa unexpectedly vetoed a plan Monday that would have allowed developers to increase the height of commercial and residential high rises by using 9 million square feet of unused development rights above the low-slung center.

In only his second veto in nearly two years, Villaraigosa told the City Council in a message that he supports an air rights initiative but as the law behind the plan stands now, it violates the City Charter because it gives no opportunity for a standing mayor to reject any projects that may be part of the plan.

The City Attorney’s Office, which approved the ordinance, said that it stands behind its decision that law does not violate the charter. And Councilwoman Jan Perry, who represents the area, said she was perplexed because the mayor’s office had months to weigh in on the issue and had not previously raised such a concern.

Some City Hall observers thought they saw politics behind the move, as the plan has generated controversy, with critics alleging little thought was given to the impact the higher density high rises would have on traffic congestion.

Perry has said she was unsure whether she will tweak the ordinance to meet the mayor’s concerns, or instead seek to have the council override the veto. Ten votes of the 15 member council are needed for an override.