Travelers walk through Miami International Airport on Wednesday.  Lynne Sladky/Associated Press

WASHINGTON—If the number of consumer complaints filed with the U.S. government is any measure, air travel got more miserable last year.

The Transportation Department said Friday that it received nearly 97,000 complaints in 2023, up from about 86,000 the year before. The department said there were so many complaints that it took until July to sort through the filings and compile the figures.

That’s the highest number of consumer complaints about airlines since 2020, when airlines were slow to refund customers after the coronavirus pandemic shut down air travel.

The increase in complaints came even as airlines canceled far fewer U.S. flights – 116,700, or 1.2% of the total, last year, compared with about 210,500, or 2.3%, in 2022, according to FlightAware data. However, delays remained stubbornly high last year, at around 21% of all flights.

So far this year, cancellations remain relatively low – about 1.3% of all flights  – but delays are still running around 21%.

More than two-thirds of all complaints last year concerned U.S. airlines, but a quarter concerned foreign airlines. Most of the rest concerned travel agents and tour operators.

Complaints about treating passengers with disabilities rose by more than one-fourth compared with 2022.

Complaints of discrimination, while small in number, also rose sharply. Most were about race or national origin.

Airlines receive many more complaints from travelers who don’t know how or don’t bother to complain to the government, but the carriers don’t release those numbers.

The Transportation Department is modernizing its complaint-taking system, which the agency says will help it do a better job overseeing the airline industry. However, the department has released complaint numbers many months late. It did not issue figures for the second half of 2023 until Friday.