A 'system outage' delayed flights across all airlines at London's Heathrow Airport last night and this morning.
Computer systems at the UK's largest airport are still being "monitored closely", a spokesperson for Heathrow said.
Last night airline staff were forced to manually input baggage entries, causing a backlog which left some people sitting in the departures lounge until the early hours of Monday morning.
Further, many passengers were left without their luggage when they finally reached their destination.
One passenger, Keith Walker, tweeted: "@British_Airways landed Singapore Sunday. Still no luggage or even traced on your website. Any ideas? #notlookinggood"
British Airways responded: "I'm sorry your bags were delayed by Heathrow's system problems. Please DM [direct message] your name and tracing ref [sic] and I'll have a look."
Heathrow would not give any further details about the cause, or how many passengers were delayed. Airlines including Aer Lingus and British Airways announced that all airlines had been affected, throughout all terminals, indicating that the total number of passengers delayed could be in the thousands.
A Heathrow spokesperson said: "There was an IT issue at Heathrow last night and early this morning which affected part of the baggage system. A contingency was put in place to complete this process manually, but some passengers have travelled without their bags.
"We are very sorry to those passengers who have been affected by this and are working with airlines to reunite people with their bags quickly.
"IT systems are operating this morning and we are monitoring closely."
In July, Heathrow suffered a similar "technical glitch" that left thousands without their luggage.
Heathrow's CEO John Holland-Kaye was forced to apologise following the chaos on his first day after taking over from Colin Matthews, who now leads the Highways Agency, the government body that runs the UK's roads.
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