One third of flights which had been due to use Stansted Airport on Thursday were cancelled due to the national security alert.
Passengers are being asked to be patient when facing delays
Armed police blocked the drop-off point and passengers were moved into the short-stay car park so officers could control access to the terminal.
Airport officials later said they were anticipating people staying in the airport overnight.
Hire car firms and local hotels said they were fully booked.
"It is quite possible that we will have people staying in the airport overnight," a Stansted spokesman said.
"We will have extra staff on this evening and during the night to make people as comfortable possible.
"Aircraft are leaving and arriving and it all depends on how many people stay in the hope of rebooking flights tomorrow (Friday)."
He said delays would be expected on Friday and he urged passengers to check with airlines before setting out from home.
Easyjet cancelled all flights from Stansted Airport on Thursday due to the congestion.
Ryanair said many of its flights were hit by delays.
Spokesman Peter Sherrard told BBC News that delays looked less serious at the other airports used by Ryanair.
Many holidays have been ruined by the delays
"But there are significant delays at Stansted and there will certainly be more delays and possibly cancellations," he said.
Problems for passengers travelling to the airport were compounded on Thursday as traffic on the M11 at the Stansted turnoff suffered serious delays because of a serious accident.
Alternative healer Jayne Lewis, of Lincoln, was waiting for a Ryanair flight to Turin where she was visiting family.
"It is irritating but I think there is no way of avoiding it. It has to be done.
"Hopefully the police know what they are doing. I suspect this is all a bit over the top but that is probably the way it has to be."
A British Airport Authority spokesperson said all passengers must be hand searched, and their footwear and all the items they are carrying must be X-rayed.
The security moves follow reports that a plot to blow up planes from the UK to the US has been foiled.
Security at all airports in the UK has been tightened after MI5 raised the UK terror threat level to critical - the highest possible.