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International airline passengers arriving at Heathrow Airport near London and at other airports in Britain were facing long lines at immigration checkpoints on Tuesday evening after the nation’s Border Force reported a nationwide computer outage, officials said.

“Border Force is currently experiencing a nationwide issue which is impacting passengers being processed through the border,” Heathrow, the country’s largest airport, said on social media at 9:15 p.m.

The problem was also affecting Manchester Airport, Gatwick Airport near London and Belfast International Airport in Northern Ireland. The overall number of affected airports wasn’t clear.

Officials said that the problem originated with the Border Force’s “eGates,” immigration checkpoints that process arriving passengers through the border.

Officials at Gatwick and Manchester Airports said that there were no flight delays or cancellations related to the outage, but said that they were working with Border Force officials to assist passengers experiencing delays at immigration.

It was unclear when the outage began or what specifically caused it. An inquiry emailed to the U.K.’s Home Office, which administers the Border Force, was not immediately returned Tuesday.

Images on social media showed crammed, stagnant lines in front of a row of gates beneath signs that read “UK Border” at Heathrow Airport.