A new government study concludes there was relatively little decline in competition on U.S. air routes during a five-year period of major airline consolidation, a finding that it attributes in part to the rapid expansion of discount carriers.

The Government Accountability Office report, requested by Congress and published on Wednesday, examined the effect on competition from 2007 to 2012, a period of three major mergers creating today's Delta Air Lines Inc., United Continental Holdings Inc. and Southwest Airlines Co.

On the 37 most-traveled U.S. routes, traversed by about 83 million fliers a year, the number of airlines providing nonstop or connecting service decreased to an average of 4.3 in 2012 from 4.4 in 2007, the GAO found. The report counted only airlines with more than 5% market share on the given routes. On the 9,379 smallest city pairs, also traveled by 83 million passengers, the average number of competitors decreased to 3 in 2012 from 3.3 in 2007.

Measuring airline competition is complicated, with prices and other variables fluctuating frequently. This and other studies have found that, despite a generally level number of competitors on most routes, U.S. airlines cut their domestic flights by 14% and increased domestic fares by 4% between 2007 and 2012.

The GAO tied fare hikes to service cuts across the country, particularly in small to medium-size cities, which have lost the most flights in recent years.

The GAO cautioned that consolidation's effects are still playing out. Integration at some merged companies is continuing and the study didn't include the December union of AMR Corp. and US Airways Group Inc. to form American Airlines Group Inc., now the largest carrier by traffic. That deal left the four biggest airlines controlling about 83% of the domestic seats in the U.S.

The GAO, Congress's investigative arm, said that markets maintained competition in part because low-cost airlines expanded rapidly into busy markets. The average number of discounters in each of the 37 busiest city pairs increased to 2.3 in 2012 from 1.7 in 2007, the report said.

The GAO also said that while mergers can eliminate a competitor on many routes, they also can create new connections. For example, the 2010 United-Continental merger created a new option for fliers between Fargo, N.D., and Amarillo, Texas, via a stop in Denver, the report said.

Despite the expansion of low-cost airlines, those discounters are also doing a worse job of suppressing fares than in past years, the GAO said. In particular, Southwest "no longer seems to have the price disciplining effect it once had," it said.

Mergers and capacity cuts have also increased the number of markets dominated by a single airline. One airline carried at least half of the passengers on about 77% of all city pairs in 2012, up from 72% in 2007, the study found.

However, the number of markets served by only one carrier decreased by about 9% to 1,566 city pairs over the period, though virtually all of those markets are among the least traveled in the nation, the GAO said.

Write to Jack Nicas at jack.nicas@wsj.com

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