Heathrow Airport has been evacuated as passengers face huge delays.
Some areas of Terminal 3 have been temporarily evacuated while the fire service investigates a fire alarm.
Staff are working as quickly as possible to resolve this, according to the airport's social media.
Heathrow has apologised for any disruption this may cause to journeys.
HEATHROW AIRPORT | Heathrow Airport has been evacuated as passengers face huge delays
Passengers have reported huge queues and baggage claim being shut.
Flyers questioned why immigration lines were "not moving" at the airport.
One upset passenger wrote: "Can someone tell us why immigration lines at Heathrow are shut down?
"Been standing here for an hour with no movement in sight."
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|Passengers have reported huge queues and baggage claim being shut
Another said: "Border shut down been waiting here forever.
"Why is there not a contingency plan for a situation like this?
"We can’t stay here forever…"
A third added: "Fire alarm in baggage claim which is where we exit from passport control. All sealed off for now."
PA | Heathrow has apologised for any disruption this may cause to journeys
GB News now understand passengers are being allowed back into the terminal and reunited with their bags.
A Heathrow spokesman said: "Following an earlier fire alarm evacuation in parts of Terminal 3, the incident has now been stood down.
"We apologise for any disruption caused to journeys."
Europe's biggest airport was highly criticised earlier this year after a fire at an electrical substation nearby resulted in around 1,300 flights being cancelled, leaving at least 270,000 passengers facing travel disruption.
This comes as this weekend has been deemed to be "one of the busiest of the year" by travel association ABTA.