LONDON -- Scores of flights were delayed or cancelled at Britain's busiest airports Saturday because of a technical problem at a main air traffic control centre.

National Air Traffic Service, which controls the crowded airspace over England, said it was having a problem switching between nighttime and busier daytime operations at its control centre in Swanwick, southern England.

"This may result in some delays, for which we apologize," the centre said, but insisted safety had not been compromised.

It said engineers were trying to fix the problem, but gave no time estimate. The Association of British Travel Agents said the problem was unlikely to be resolved before 1400 GMT.

Heathrow Airport -- Europe's busiest -- had delays of an hour or more on many morning departures, and cancelled several dozen short-haul flights to destinations including Nice, Milan and Istanbul.

London's Stansted airport said departing flights were delayed between 30 minutes and two hours because of restrictions on air space and the flow of planes. Gatwick airport said 20 per cent of flights were being delayed.

Spillover delays were reported across Britain and at Dublin airport in Ireland.

The air traffic service normally handles up to 6,000 arriving and departing flights a day from its bases at Swanwick and at Prestwick in Scotland.