Passengers at the UK's biggest airport could face delays tomorrow as air traffic control is switched to a new £50m control tower.

The switchover at Heathrow Airport is timed for 2am on Saturday April 21 and follows two years of planning.

National Air Traffic Services (Nats) said it was reducing the number of landings while controllers familiarised themselves with the new tower.

Nats general manager Martyn Jeffery added: "I'm confident they will quickly build familiarity. Nonetheless, at an airport used as intensively as Heathrow, this means it is possible there will be some delays to flights although we are working very closely with (airport operator) BAA, and with British Airways, bmi and other airlines, to make sure any delay is minimal."

The new tower is 285ft (87m) high and and is key to the start of operation of Heathrow's new Terminal 5, which opens in March 2008.

The new tower replaces the existing tower, which was opened in 1955 having been designed by famous architect Sir Frederick Gibberd.

The old tower is 125.6ft (38.3m) high and replaced the original RAF control tower.