A BA statement, cited by CNN, read: Our IT teams are working as hard as they can to quickly fix a problem with our check-in system.
"We are checking-in customers this morning as normal", they told Twitter users, "although it may take longer than usual".
British Airways has apologized to passengers after an unspecified technical issue impacted its check-in computer systems.
One passenger posted a notice by BA on Twitter confirming that passengers would be delayed for up to 30 minutes while alternative arrangements were put in place "in the absence of the computer system".
The airline said it was operating a limited number of services from Dublin and Amsterdam to London Southend Airport, with complimentary rail connection to London, to facilitate passengers disrupted by the actions at London City Airport.
"We are sorry for the delay to their journeys", it said. Waiting at ORD for @British-Airways 296.
The glitch affecting BA follows the crippling power outage suffered by Delta Air Lines last month, although it doesn't appear as severe.
British Airways says it has fixed a computer problem that caused delays and long lines at numerous airports.
Planes were reportedly grounded at global airports including London Heathrow, London Gatwick, and Manchester in England, as well as Toronto, Atlanta Chicago and Seattle in the US.
"We're now experiencing disruption to all flights due to protesters at the airport", City Airport said on Twitter.
At London City airport a group of nine activists from the Black Lives Matter campaign group chained themselves to a tripod positioned on the single landing strip, causing the terminal favoured by corporate travellers to shut to traffic.
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