WHEN the Queen opened Heathrow’s £4.3 billion terminal building earlier this year, it was supposed to herald a new era in state-of-the-art travel.

Instead, the launch of terminal 5 rapidly descended into farce, with dozens of flight cancellations, enormous check-in queues and thousands of items of luggage mislaid.

The chaotic scenes in March served only to exacerbate Heathrow’s reputation for overcrowding and misery.

“The bottom line is that it’s a third-world airport,” said one former airline boss. “It’s a national disgrace.”

Boris Johnson, the London mayor, has described Heathrow as “a planning error of the 1960s”.

Now his officials are drawing up proposals to close it and replace it with a 24-hour airport located on an artificial island in the Thames estuary.

“If you look at what is going on in other countries around the world - in Hong Kong, in Washington - it’s not impossible to move the capital’s biggest airport,” Johnson has said.

It may at first sound implausible, but proposals for an airport in the Thames have endured for almost 40 years.

Johnson favours a four-runway hub off Sheppey in Kent, which could easily be expanded to six runways because of minimal planning constraints.

The airport would be connected to the high-speed Channel tunnel rail link to transport passengers into central London in about 35 minutes. And the Continent would be just a short train ride away in the opposite direction, cutting out the need for many shorthaul flights.

Officials at London’s city hall believe the airport could be built in as little as six years and ultimately envisage Heathrow being closed and turned into a high-tech business and residential development.

“I think it’s madness to expand any of the other airports when there is an obvious solution elsewhere,” said Kit Malthouse, one of Johnson’s deputies, who is overseeing the Thames airport project.

“We’re not proposing to switch the lights on at the new airport and switch the lights off at Heathrow, firing everyone overnight. This would be a phasing from one airport to the other. Over the space of three or four years, those [workers] that wanted to, could migrate.”

Johnson’s team have conducted a preliminary review and now plan a more detailed feasibility study, involving an engineering consultancy. Hong Kong’s island airport, which opened in 1998 with two runways, cost £10 billion.

Mayoral advisers recently outlined some of their plans to Sir Richard Branson’s Virgin Atlantic airline. Sources close to Johnson say the carrier expressed an interest in the Thames project.

However, Virgin this weekend denied it was prepared to provide any financial support or move its fleet to the new hub.

The airline regards a new runway at Heathrow as a priority to alleviate congestion. “We will await the results of the study with interest,” said a Virgin spokesman.

Last week Zhang Mao, the deputy mayor of Beijing, indicated to Johnson that he would consider investing in infrastructure projects in London, including venues for the 2012 Games. Johnson will discuss financing of the Thames estuary airport when Zhang travels to the capital next month.

Malthouse believes the most sensible location for a new airport is about two miles north of the Isle of Sheppey where the estuary is only 10ft-13ft deep. An artificial island could be created from landfill. It would be connected to the mainland by a railway bridge and ferry terminals would link it to both Kent and Essex.

Aircraft would descend over the North Sea instead of disturbing residential areas in the approach to Heathrow.

“You would have no problems with expansion or noise,” said Malthouse. “You could run a 24-hour airport.”

The government is expected to decide by the end of the year whether to allow a controversial third runway to be built at Heathrow, at a cost of up to £13 billion.

Internal Department for Transport documents, obtained under the Freedom of Information Act, show there is a “high risk” that a new runway would breach noise and air quality targets set by the European Union.

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