Michelle Thomas had another two flights to get home to the States

Across the UK, passengers to all destinations were warned to expect severe delays after the system failed for one hour - long enough to generate a sizeable backlog.

Queues were exponentially lengthening at Heathrow's Terminal 1 by mid-morning as more and more passengers turned up for their flights.

For 20-year-old American student Michelle Thomas, the expected delay of two hours was going to be cutting it fine for her next connection.

She had flown from Italy the night before, and after a night at east London's Stansted airport had travelled west to Heathrow for her 1300BST flight to Iceland.

"I'm concerned that if I get delayed, I'll miss my other connections. I'm really crossing my fingers," she said.

She has a mere two hours between arriving in Reykjavik and departing for Baltimore, US, finally returning home after a year abroad and a long air journey.

"I'm so totally exhausted sometimes I'm delirious."

London Transport was warning passengers of the delays

There was a general feeling of being resigned to the disruption as the check-in hall got progressively fuller.

London resident Charles Cruickshank said he was "quite calm" as he sat waiting to check-in for his 1250BST flight to Edinburgh for a holiday.

"I did hear an announcement about air traffic delays, but I'm hoping it will be OK," he told BBC News Online as he read his newspaper.

Another holiday-maker - off to Spain - said: "There's nothing we can do about it, so there's no point worrying."

He was told to "join the line" when he arrived to check-in for his 1130BST flight and was ready to wait out the delays.

"I've got nothing urgent to do, so I'll just wait until the flight goes."

Waiting was slightly more problematic for Finnish woman Miia Chambers.

She had accompanied her sister and two nieces - five and seven years old - for their flight home.

They had already been at the airport for about two hours by 0940BST, waiting to check-in for their 1020BST flight.

Miia Chambers was not looking forward the long wait with her nieces

"We're hoping the flight will leave today, but what is tough is keeping two children entertained, because they don't understand why they have to wait," she said as her nieces played with colouring books next to her, and her sister went in search of an update on the flight.

Ms Chambers herself was not flying, but had stayed with the family because her sister's English was not fluent.

"I might have to take a day off work, but I'll check with the boss shortly."

The international arrivals hall of Terminal 1 was almost empty compared to the departure hall.

The information screen showed the delays to arrivals, including a flight from Helsinki due to arrive one hour 47 minutes late, and one from Madrid due one hour 45 minutes late.

But it was not all bad news - the 1004BST arrival from Athens had actually arrived 20 minutes early, according to the screen.