Flight delays and cancellations rose as competition between carriers declined at U.S. airports, according to a study ordered by Congress in the wake of airline mergers.

The relationship between the average length of a delayed flight and competition was “statistically significant and sizable,” the U.S. Department of Transportation’s inspector general found.

“When competition decreased, both the average length of flight delays and percentage of total flights that were late increased,” according to the report by Mitchell Behm, an assistant inspector general.

Since 2005, mergers have winnowed the number of major carriers to four: American Airlines Group Inc., United Continental Holdings Inc., Delta Air Lines Inc. and Southwest Airlines Co.

The length of time a flight was delayed increased by 25.3 percent when a market’s airline service shrank from three to two, the study found. The impact on the percentage of late flights was smaller and barely statistically significant, it said.

A market that went from three carriers to two also had a 7 percent increase in canceled flights, according to the study.

The study examined more than 2,500 U.S. routes flown by 20 airlines from 2005 through 2012. The IG developed economic models to estimate the effects of competition on delays and cancellations.

To contact the reporter on this story: Alan Levin in Washington at alevin24@bloomberg.net. To contact the editors responsible for this story: Bernard Kohn at bkohn2@bloomberg.net.

Photo Credit: Delta planes line up at their gates while on the tarmac of Salt Lake City International Airport in Utah. Lucas Jackson / Reuters