A bomb disposal police unit has been sent to Gatwick Airport after a “suspected prohibited item” was found in luggage.
Sussex Police said a “security cordon has been put in place” inside the south terminal of the UK’s second busiest airport.
A large part of the building has been evacuated.
Around 100,000 passengers are set to fly in and out of the airport on Friday, but many are likely to be missing their flights.
Flights continue to depart from the south terminal, but are only carrying passengers who had passed through security before the incident began.
Sussex Police said in a statement: “Police were called to the south terminal at Gatwick Airport at 8.20am on Friday following the discovery of a suspected prohibited item in luggage.
“To ensure the safety of the public, staff and other airport users, a security cordon has been put in place whilst the matter is dealt with.
“As a precaution, an EOD (explosive ordnance disposal) team is being deployed to the airport.
“This is causing significant disruption and some roads around the south terminal have been closed.
“We’d advise the public to avoid the area where possible.”
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Some flights have departed from the south terminal since the evacuation, meaning some passengers may have been unable to board.
A British Airways flight to Cancun, Mexico, departed at 11.16am, 26 minutes behind schedule.
Several British Airways departures are delayed by at least one hour and 40 minutes, including to Orlando, USA; Funchal, Portugal; and Tenerife, Spain.
Spanish carrier Vueling ordered an inbound flight from each of Barcelona and Seville to turn around and return to their starting airports.
A Norwegian flight to Bergen departed two hours and seven minutes late.
A spokesman for Gatwick said: “A large part of the south terminal has been evacuated as a precaution while we continue to investigate a security incident.
“Passengers will not be able to enter the south terminal while this is ongoing.
“Safety and security of our passengers and staff remains our top priority.
“We are working to resolve the issue as quickly as possible.”
The south terminal at Gatwick is affected (Alamy/PA)
The north terminal is unaffected.
Nejadeen Braham, 35, who works as a nurse on the Isle of Man, was due to fly to her homeland of Jamaica to collect her children.
Speaking to the PA news agency outside the south terminal, she said: “I couldn’t get in the airport. I saw everybody coming (out) from one direction.
“I was supposed to go inside, drop my bags and go through security.
“When I got here I saw everybody coming down.
“There were no proper updates.
“I m very frustrated.”
British Airways said in a statement it is “in contact with affected customers and giving them updates as and when we can”.
It went on: “We’re also providing them with options including flight re-bookings and hotel accommodation where necessary.
“We ask customers to check ba.com for the latest flight information before coming to the airport.”
Train services at Gatwick have been suspended, as the railway station is attached to the south terminal.
Passenger information website National Rail Enquiries said: “The police are dealing with an incident at Gatwick Airport. The station and airport are currently being evacuated.
“Services are able to run through the station and area but will not call at Gatwick Airport at this time.
“Please avoid travelling to the station whilst this incident is ongoing.
“This is expected to continue until approximately 2pm.”
Aviation analytics website Cirium said 316 flights are scheduled to depart from the airport on Friday, with a total of more than 60,600 seats.
A further 318 flights are scheduled to arrive, with more than 61,400 seats.