United Airlines battled service delays this weekend as it worked through technology glitches during its combination of the United and Continental Airlines reservation systems.

United on Saturday adopted the reservation platform of the former Continental Airlines after the companies merged to form the world's largest airline, now known as United Airlines and owned by United Continental Holdings Inc .

But after spending months preparing for the change -- including training about 15,000 employees on the new software -- United said on Sunday that technical issues had flared up at airports across the system, causing delays.

In particular, problems with the company's airport check-in kiosks meant customers had instead to line up to see service agents, spokeswoman Megan McCarthy said.

We did have some issues with our kiosks and at times that slowed the check-in process, McCarthy said, adding that the airline's performance had improved by Sunday morning as the company installed upgrades to the kiosks.

About 75.5 percent of United's mainline flights were on time -- arriving within 14 minutes of their scheduled slot -- on Sunday morning, as were about 87.1 percent of its Express flights, according to McCarthy. Employees are working hard to take care of our customers, she said.

Migration to a single reservation system comes with risks, as US Airways Group learned in 2007 when it attempted to combine the reservation systems of the former America West Airlines and US Airways. The two airlines merged in 2005.

A glitch in combining that system caused self-service kiosks to fail and forced passengers to stand in extraordinarily long lines. Thousands of travelers had to wait and about 500 at the Charlotte, North Carolina hub missed flights.

Delta Air Lines had a much smoother transition to a single reservation system following its 2008 merger with Northwest Airlines.

Since the United transition, customers going to the Continental.com website are now directed to the United website.

On Sunday the website said its transition is almost complete but it noted that the company was having some technical issues.

The vast majority of this work is going well, and we are resolving technical issues that we are identifying during this process, a statement said.

(Reporting by Sinead Carew; Additional reporting by Kyle Peterson in Chicago; Editing by Dale Hudson)

United airlines