Complaints about poor airline service nearly tripled in May from the same month last year, the Transportation Department said yesterday.

The number of complaints last month, 2,816, also rose 34 percent from April. The statistical confirmation of rising consumer dissatisfaction appeared likely to add to the rising pressure on the airlines to clean up their acts.

"These numbers are appalling. The mistreatment of consumers has got to stop," said Rep. James J. Florio (D-N.J.), chairman of the House Energy and Commerce Committee's subcommittee on consumer protection. "If it's this bad in May, what will it be like in the summer travel season?"

The biggest offender, according to DOT's monthly compilation, was Continental Airlines, with 793 complaints, or 21.39 per 100,000 passengers.

Next highest was Eastern Airlines -- which, like Continental, is a subsidiary of Texas Air Corp. Eastern had 445 complaints, or 10.11 per 100,000 passengers. Delta Airlines had the lowest level of consumer complaints, registering 1.78 per 100,000 passengers.

However, a spokesman for Continental noted that the number of complaints about the airline was down from the month before. "We're pleased that the trend seems to have leveled off and be headed downward," said Rick Scott. In April, Continental received 25.4 complaints per 100,000 passengers.

Scott said the airline is still suffering the consequences of its consolidation earlier this year. In February, Continental absorbed People Express and New York Air as well as some of the assets and operations of Frontier Airlines.

The consolidation put enormous pressure on the airline, particularly at its hub in Newark. During some periods early in the consolidation only about a third of the departures from that airport were on time, said Scott. Now the airline is getting about 85 percent of its flights off on time, he said.

"We believe that over several months or so these numbers will come down because they do not reflect the activities of the airline today," Scott said. Some of the complaints filed in May reflect problems that occurred earlier, he said.

The largest number of complaints filed with the DOT were over flight problems, a category that includes delays, overbookings and cancellations. Baggage problems ranked next, followed by refund problems.

Consumer complaints have prompted calls for legislation. Last month, Transportation Secretary Elizabeth Hanford Dole warned the airlines of possible federal action unless the situation improved.

Airlines increasingly are competing not just in fares but in consumer service. Continental and American Airlines have proposed that airlines be required to provide information about service to allow consumers to shop for service as well as prices.

Christoper J. Witkowski, executive director of the Aviation Consumer Action Project, a Ralph Nader-affiliated organization, said yesterday that increased attention on complaints may have generated some increase in the numbers, but he said that most of the rise reflects an increase in problems. "That's a tremendous increase just from April to May, which shows that more passenger problems are being experienced," he said.