Stansted Airport has apologised after a fault with security scanners caused long queues and meant some passengers missed their flights.
The problem with the X-ray machines emerged at around 4.30am on Tuesday and was resolved by 8.15am, a spokeswoman for the Essex airport said.
Passengers posted photographs of large crowds inside the terminal building on social media.
One traveller, posting on Twitter with the username Kate-Elizabeth, wrote: "All the scanners went down at Stansted Airport.
"There was a huge pile up, staff didn't prioritise the times of anyone's flights and Ryanair didn't hold the flight for all those delayed.
"It was still on the tarmac when we all eventually got to the gate only to be told we couldn't board.
"Missed flight so now in another huge queue with loads of others trying to get another one. All staff completely unhelpful."
She added she was "quite honestly doing my best not to cry this morning".
Another passenger, Meagan Tetro, tweeted: "Hundreds stranded at Stansted Airport due to their system failing. All missed flights and now stuck queuing."
She went on to say she was in a queue with about 150 other people needing new flights, and described the situation as an "embarrassing show of service" from the airport and Ryanair.
A Stansted spokeswoman said: "Due to a technical issue earlier this morning with a number of X-ray machines in the main security area, throughput capacity was reduced, taking extra time to process passengers.
"Engineers rectified the issue and additional measures were put in place to minimise the impact to passengers and assist them with their onward journey.
"We apologise for the inconvenience caused. The area has now returned to normal."
Stansted is the fourth busiest airport in the UK and is used by an average of 67,000 passengers each day.
A Ryanair spokesman said: "While we regret any inconvenience caused by this power failure at London Stansted Airport, Ryanair is not responsible for airport security delays."