Delta Air Lines has been fined $750,000 for bumping passengers from oversold flights without compensating them or seeking volunteers.

It is the second time in four years that the airline has been penalised over the issue. Delta was fined $375,000 in July 2009.

US transportation secretary Ray LaHood said: “Airline passengers deserve to be treated fairly, especially if they are forced to miss a flight because an airline oversold seats.”

US Department of Transportation investigators found “numerous instances” of violations in a review of 310 customer complaints at Delta’s headquarters, according to a Bloomberg report.

But Delta told the department the customer complaints that led to the fine were sometimes ambiguous or incorrect.

A spokesman described the violations as “an isolated situation, which represent a tiny fraction of the passengers that we handle”.

Delta will be allowed to spend as much as $425,000 of the fine on improving how it complies with the regulations governing bumped passengers.

The airline agreed to purchase tablet computers to allow gate agents to better keep track of passengers who were denied boarding.

“We’ve invested a considerable amount in technology that is designed to improve how we handle oversold flights,” a spokesman said.

If an airline has sold more seats on a flight than are available, US rules require that it seek volunteers willing to wait for another flight before removing, or “bumping,” passengers.

Passengers who are bumped are entitled to as much as $1,300 in compensation.

Delta classified some passengers it bumped as having willingly given up their seats, which undercut their right to compensation.