Of all the airlines that took a hit in the annual American Customer Satisfaction Index (ACSI) report, United was dinged the most.

Now the carrier is fighting back.

Responding to a TravelPulse.com query, United says it has invested significantly to improve customer service despite the drop in rating which left it with an ACSI score of 60 (out of 100), the lowest among the major airlines by six points and a 3 percent decline from last year.

“United is investing significantly to provide employees the tools to deliver great service and improve our customers’ experiences,” spokesman Rahsaan Johnson told TravelPulse. “While we have reduced complaints year-over-year and improved on-time performance and overall reliability, we continue to develop new customer products and services and employee programs to deliver the experience travelers expect.”

According to the ACSI report, United is still struggling with passenger service following its 2010 merger with Continental and the study expressed concern going forward.

“Since United is so far behind other airlines in passenger satisfaction, it will not be easy to gain market share or to keep customers, especially when considering that 30 percent of their passengers have an ACSI score below 50,” the study stated.

 “We’ve seen time and time again the negative impact mergers have on customer satisfaction. American Airlines may also see a slump in satisfaction as it combines operations with US Airways,” said Claes Fornell, ACSI Founder. “Southwest led the industry for 17 years until it merged with Airtran in 2011 and Delta is just now recovering from its 2008 merger with Northwest.”

Johnson said among United’s continuing improvements:

• Completed the first round of customer-service training for all customer-facing employees
• Completed installation of updated international premium cabins with flat-bed seats in first and business
• Deployed satellite Wi-Fi on 200 aircraft (most of the remaining aircraft to be completed this year)
• Increased overhead bin storage on scores of aircraft, particularly those that fly in domestic markets
• Opened completely redesigned airport lounges in Chicago, Seattle and San Diego (two more upcoming between now and June).
• Launched an all-new mobile app that gives customers additional information and functionality

Overall, since ACSI first established baseline scores in the summer of 1994 for the six major airlines (JetBlue, Southwest, Delta, American, US Airways and United), as well as an "all others" category, every single airline has either been flat or has fallen in customer satisfaction.

United took the biggest drop at 15.5 percent.