British airports were forced to delay and cancel flights
Flights to and from some UK airports have been cancelled or delayed because of a computer problem at the main air traffic control centre at Swanwick.
Airports across the UK have been affected by delays of flights using airspace in the south-east of England.
The problem has now been fixed and although services are said to be returning to normal, some delays are still occurring.
British Airways cancelled 35 domestic and European flights from Heathrow.
The computer problem occurred at 1600 BST and it is not certain how much of a backlog has built up and what knock-on effects the incident will have.
Simon Calder, travel editor at the Independent newspaper, told the BBC there would be repercussions for travellers tomorrow.
"There will be an impact because crews and aircraft are going to be out of position," he said.
"So many cancellations today, passengers will be wanting to re-book, and it's always the short-haul cancellations, Paris and Brussels and Scotland that get hit."
Passengers at airports across the UK, including Heathrow, Cardiff, Belfast and Glasgow, faced delays.
British Airways said most short-haul flights were now operating "almost normally" at Heathrow.
At Heathrow's Terminal 5, Richard Minnis, 26, an IT consultant from London, missed his 1910 flight to Cape Town after getting stuck in queues caused by the Swanwick incident.
"It has been a bit of a shambles really," he said.
"I missed my flight because the queue was so long to check in. Now I am stuck trying to find another one and the queues are huge."
At Gatwick, Eamon McGee, a 36-year-old construction consultant from Northern Ireland, had planned to catch an Easyjet flight to Ibiza for a stag party.
"There's a queue of about 100 people waiting to speak to someone," he said.
"There hasn't been any information, I've only heard things from what's on the television in the restaurant."
National Air Traffic Services (Nats) said engineers were called in to investigate the cause of the computer fault at the London Area Control Centre, which deals with planes flying over south-east England.
The London Terminal Control Centre, also based in Swanwick which is responsible for landing and departing aircraft, continued to operate fully but it had been restricting departures due to the additional workload.
LONDON AIR TRAFFIC CONTROL
The London air traffic control centre at Swanwick, Hampshire, has two parts with different responsibilities:
The London Area Control Centre (LACC) is responsible for aircraft flying over England and Wales
The London Terminal Control Centre (LTCC) handles aircraft approaching and departing south-east England airports
Both centres are run by National Air Traffic Services (Nats), which is part-owned by the government and a consortium of airlines
Nats said restrictions on take-offs and landings had been introduced to ensure passenger safety while controllers operated manual systems at reduced capacity.
BBC transport correspondent Tom Symonds said the problems stemmed from a glitch with computers which deal with information about flights when they are at higher altitude.
Air traffic controllers could still see where planes were, but were finding it difficult to identify them, he said.
This had resulted in a reduced flow of aircraft from airports, with planes at Heathrow leaving every two minutes rather than every 90 seconds.
Luton Airport cancelled nine European flights but its website said while delays remain on some departures, check in for the majority of flights had resumed and services are returning to normal.
Flights from Cardiff International Airport were also temporarily halted by the fault, but had started moving again a spokesman said.
A spokesman for Manchester Airport said some European and international flights had been affected as they were routed to fly over south-east of England.
A spokesman for airport operator BAA, which runs Heathrow, Gatwick, Stansted, Glasgow, Edinburgh, Aberdeen and Southampton airports, said "many UK airports" had been affected by the problem.
"Aircraft continue to land and depart, however the process is slower than normal, which means that inevitably, some flights will be delayed and some will be cancelled," he said.
"We are working hard with the airlines to minimise disruption."
About a dozen flights were cancelled from BAA's three Scottish airports, including flights to Heathrow, Gatwick, Madrid and Alicante.
A spokeswoman also said several flights from Scotland had delays ranging from 15 minutes to two hours, a spokeswoman said.
Airport authorities have advised passengers to contact their airlines before travelling.