A decline in air travel delayed plans for a second control centre

The new centre at Prestwick in Ayrshire is key to the "two centre" strategy, sharing responsibility for flights through UK and North Atlantic airspace.

An advantage which had been cited for the plan was to provide back-up in the event of system failure.

But work at Prestwick was suspended for two years after the 11 September attacks on the United States.

Fear of terrorism contributed to a decline in international air travel and it was decided that the need for a second centre was less pressing.

But last month National Air Traffic Services announced the start of work at the Prestwick site.

After surveys and ground work, the ground slab is expected to be installed during the summer, with the main building work beginning later in the year.

Construction plan

But the structure will not be completed until 2006 and fitting out of the centre will then begin.

Following the two-year delay, the new Prestwick centre will not now become operationally live until 2009.

The most recent air traffic control problems have arisen from a computer failure at West Drayton.

When the current reorganisation is complete, the London Terminal Control Centre at West Drayton will move to Swanwick.

Speaking on the BBC's Good Morning Scotland programme, the Transport Secretary Alistair Darling said: "The Swanwick system is working, but West Drayton, which is the older computer system, still handles a very large number of flights coming into UK airspace, particularly coming in to the London airports.