LONDON (CBSNewYork/AP) — British Airways has apologized for delays caused by computer glitches in check-in systems and warned that there may be further troubles.

Travelers took to social media globally to complain of long lines and the airline said “a number” of airports were affected. Pictures posted on Twitter Monday night showed long lines at airports in Atlanta and San Francisco.

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The airline said passengers are being checked in at Heathrow and Gatwick Airports but that the process may be “a bit slower than usual.”

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BA had to apologize in July after computer glitches in check-in systems also delayed passengers. It began installing new systems last October and completed the rollout earlier this year.

BA says it would “encourage customers to check in online before they reach the airport.”

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