It is just a brief window in time, from 11:30 p.m. on Sunday 5 July to 6:30 a.m. Monday 6 July.

In that seven hours, Hong Kong will carry out the most critical part of the operation to move the world's third busiest international passenger airport and the busiest international cargo airport some 30 kilometres to the west.

It will be the biggest operation Hong Kong has ever seen, involving more than 1000 vehicle movements, flights by 30 jet airliners, 70 barge sailings, thousands of tonnes of equipment and l,000 policemen to control the traffic.

The time is dictated by the schedule of the last international flight to leave the existing airport at Kai Tak, at about 11:30 on the night of 5 July and the first flight to land at the new Hong Kong International Airport at Chek Lap Kok on Lantau Island, at about 6:30 on Monday morning.

So essential is Hong Kong as the region's airline hub in that there can be no interruption of the hundreds of flights that land and take off on any given day. Kai Tak must close down and the new airport open up right on time. It has to be a seamless move.

It may be the biggest peace-time operation, but it is being planned as meticulously as the D Day invasion of Normandy. It has to be to ensure that nothing goes wrong. There have been big airport relocations before, such as those at Munich and Denver. But, never on this scale; never by land, sea and air, and never through some of most densely populated areas and heavily used roads in the world.

To ensure that nothing does go wrong, the move is being planned like a military operation. It has also been divided into three stages, the Main Body Move, which will take place from 5 to 21 June, the Night Move and the Follow On Move from 6 July to 5 August. The three stages will involve a total of l0,000 vehicle movements, the most crucial being the l,000 on "The Night", as it has come to be know in Hong Kong.

Anyone who has landed at Kai Tak, with its spectacular approach over high-rise apartment blocks, will realise that vehicles moving to the new airport will have to pass through some of the densest urban areas anywhere.

They also have to pass across the Tsing Ma Bridge, one of the world's longest suspension bridges, built as part of the Airport Core Programme. It carries the road and rail links between the urban areas and the new airport.

The problems connected with the move are compounded by the nature of some of the equipment to be used, the large, strange-shaped, utilitarian vehicles familiar on airport parking areas, but never seen on roads.

Many of the vehicles, such as aircraft steps and other service machines, are not even licensed for travel on roads and have to be given special permits for the move. The size of some of the loads also presents problems; loads like the main deck loader which is 4.9 metres high when loaded on a trailer, is 4.7 metres wide and weighs 48 tonnes.

Cathay Pacific, the Hong Kong airline, knows it can successfully move loads like this between Kai Tak and the new airport because it has already had a dress rehearsal for the move. On 29 April a 40-vehicle convoy made the journey from the old to the new airport to test loading methods, the route, the police escort and entry to the new airport.

Cathay's relocation manager, James Ginns explained: "The success of all stages of the move to the new airport depends on teamwork. The trial run was co-ordinated with the Airport Authority, the Civil Aviation Department, the Hong Kong Police Force, the New Airport Projects Co-ordination Office, the Transport Department, Highways Department, the Tsing Ma Bridge Authority and Hong Kong Air Terminal Services. Getting together with all the various parties to work on this was a good way to start building the team."

Loading the equipment needed the services of three 60-tonne mobile cranes. When it set off, the 40 vehicle convoy was carrying one main deck loader, three lower deck loaders, 12 tractors, one conveyor belt vehicle, one pallet pusher, 49 assorted dollies, two high loader trucks and a control van. It took the convoy one and a half hours to make the journey.

But, that was just one convoy of 40 vehicles from just one airline. On "The Night", as it has come to be known in Hong Kong's airline industry, there will be 65 airlines involved and a host of other companies whose business is at the airport.

There will be l,000 vehicle movements, not to mention heavier loads being transported by 70 barges through Victoria Harbour and up the Ma Wan Channel to Lantau. The heaviest item to be moved will be an aircraft recovery vehicle. That is big enough to haul a disabled Boeing 747 from the runway. There will also be 30 airliners making the short hop from Kai Tak to relocate. The airliners too will be carrying equipment to be transferred from airport to airport.

This will be the first and last time that late night flights will be allowed at Kai Tak. There has been an after-midnight curfew to prevent aircraft noise disturbing residents living near the airport. This has severely restricted Kai Tak's capacity. There will be no curfew at the new airport which will have 24-hour operations.

Sleep will be far from the minds of those involved in the move because nothing must go wrong on "The Night". As the Corporate Development Director of Hong Kong's Airport Authority, Clinton Leeks, said: "The world will be watching. We are planning for a seamless transfer."

"The move might not be as big as D Day, but the same kind of planning approach is being used," said Elizabeth Bosher, the Airport Authority's Planning and Co-ordinating Director. She said her staff includes retired British army officers whose experience in military logistics has proved invaluable.

The world will be watching but, the Hong Kong authorities are hoping that if Hong Kong is watching, it will be on television. The public has been asked to stay away from the roads used for the move. However, it will be broadcast throughout the night on local televisions stations.

"We hope people will stay home and watch it on TV rather than get in the way of some of the largest vehicles ever seen on our roads," said Clinton Leeks.

Everything has been planned to the finest detail but one thing that cannot be planned in advance is the weather.

July is the middle of Hong Kong's typhoon season and the tropical location makes long-range weather forecasting difficult. If a typhoon does strike, the move and the operational opening of the airport will be postponed for a week.

Typhoons aside, Hong Kong is confident that it will be all right on "The Night". It has to be.