Flights have been grounded at Bristol Airport following a power failure in the terminal.
The airport said it had engineers on site attempting to fix the problem, and that some power to the building had been restored.
It said on Twitter: " A power failure in the terminal is causing delays to flights. Engineers are working to fix the problem. We apologise to passengers affected.
"Power has been partially restored to the terminal but some systems remain affected."
Passengers with hand baggage have been told they can proceed through security while staff check flights in order of their departure time.
Some passengers were reported to have been told to wait outside the terminal building.
Paul Raymond, who was waiting to fly to Spain, told the BBC the terminal was in darkness when he arrived at 3.10am.
He said: "For a short time they were telling people just to leave the terminal building. There is nothing - no computers, no security - nothing is operating at all."
Passengers took to social media to vent their frustration at the delay.
Barry Wilkinson wrote on Twitter at 4.46am: " No power or running water at Bristol Airport, has been for 2hrs. People keep arriving so the place is filling, but no progress. Avoid!"
An hour later, Josh Baker tweeted: "Power cut in Bristol Airport has thrown a spanner in the works, supposed to boarding the plane in 15 minutes".
Lleu Williams posted a photograph of the airport packed with waiting passengers at 6.52am.
"Chaos and mass delays at Bristol Airport as a power outage cancels flights," he tweeted.
Keith Vivian Evans wrote on Twitter: "Power cut Bristol airport. 2 hour delay chaos."
A spokesman for Bristol Airport said it experienced a power cut at 3am.
"Engineers are on site and working to fix the problem as soon as possible," he said.
"Additional airport staff have been called in to assist passengers in the terminal and bottled water is being provided.
"The power failure impacted check-in, baggage and lighting systems in the terminal but all airside and air traffic systems remained operating as normal.
"Power has been restored, but some systems remain affected. Airport staff are now working on to check in delayed flights in order of scheduled departure time.
"Passengers with hand baggage only are being processed through security search.
"Bristol Airport apologises for the inconvenience to passengers and flight disruption.
"Passengers are advised to check in as normal and to be prepared for some flight delays."