Real-estate developer Extell nearly caused a major steam-main explosion while building a $2 billion luxury downtown high-rise and so should pay for the damage, Con Edison says.

The utility giant filed a Manhattan Supreme Court lawsuit “on behalf of its innocent rate-paying steam customers for the approximately $4 million that was spent to repair the damaged portion of the steam system and restore the steam system to a safe and reliable service” at 250 South St.

The suit says Extell was warned by its own engineers that its excavation work for the 800-unit tower was perilously close to Con Ed’s underground network of steam pipes.

Still, the digging caused two joints to leak.

A rep for Extell said, “We deny these allegations and it is under review.”