The Department of Transportation (DOT) fined United Airlines $1.9 million for long tarmac delays.

The department says United allowed 20 domestic flights and five international flights to sit on the tarmac for a “lengthy period of time” without giving passengers the option to get off the plane.

That is a violation of a federal statute, the department said.

“Under the DOT tarmac delay rule, airlines operating aircraft with 30 or more passenger seats are prohibited from allowing their domestic flights to remain on the tarmac for more than three hours at U.S. airports and their international flights to remain on the tarmac for more than four hours at U.S. airports without giving passengers an opportunity to leave the plane,” a statement from the department says.

This is the largest fine ever imposed by the DOT, the department said.

In response to the fine, United said it is "committed to fully meeting all DOT rules and will continue identifying and implementing improvements in how we manage difficult operating conditions,” according to a statement obtained by NBC News.