ROYAL TOUR: British Airports Authority CEO Stephen Nelson giving Britain's Queen Elizabeth a tour of the new airport terminal during its official opening yesterday. -- PHOTO: REUTERS

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The Queen and her husband, Prince Philip, toured the new facility under heavy security less than 24 hours after a major security alert disrupted flights at the airport.

A man with a backpack scaled a fence and ran onto an active runway on Thursday, in what was Heathrow's second security breach within weeks.

'A man ran towards an Emirates flight,' a police spokesman said, after television pictures showed police vehicles scrambling to deal with the situation.

'He has been removed. We believe he was acting alone.'

A controlled explosion was carried out on the man's bag, although police said there was no sign of explosives.

They added that there was no evidence the incident was terrorist-related.

They suggested that the man might have been a protester campaigning against the expansion of the world's busiest international gateway.

The BBC quoted eyewitnesses as saying that the man scaled the airport's perimeter fence, ran onto the runway and was eventually surrounded by police.

A picture broadcast on Sky News appeared to show a policeman levelling a gun at the man as other colleagues dealt with him on the ground.

The incident came just over three weeks after four activists from environmental group Greenpeace managed to bypass security and climb on top of a parked jet to protest against the planned construction of a third runway.

Heathrow is already one of the world's biggest airports, but soaring demand for air travel means that for years, its ageing buildings have been plagued by problems such as overcrowding, baggage losses and delays.

T5, which will open to passengers on March 27, is one of the first steps to address the problem and maintain London's reputation as a world business and tourism hub and financial centre.

Construction of what is Britain's largest enclosed space took nearly six years and cost £4.3 billion (S$12 billion). Some 21km of tunnels, 16km of baggage conveyor belts and a direct underground train line to central London were built.

With a floor area equivalent to 50 full-size football pitches, the airport is also fitted with gourmet restaurants, luxury shops and high-tech baggage systems.

It is hoped the new facility will ease the frustrations of the 67.3 million passengers who squeeze each year through an airport that began as a tented village in 1946.

A further £4 billion will be spent in coming years to knock down the ageing terminals 1 and 2 to make way for another new terminal.

The monarch's decision to open the facility personally - 53 years after she opened the airport's first passenger terminal - reflects the importance of the new building, which officials believe will restore the airport's faded reputation.

Airport operator British Airports Authority's chairman Nigel Rudd told the BBC yesterday: 'London deserves a first-class airport. We are starting today. This is a new beginning.'

But green protesters worry that the new terminals are just the first stage in a vast new expansion, which as well as the construction of a third runway will see an increase in passenger numbers by up to 100 per cent.

While the Queen yesterday described T5 as 'environmentally responsible', Greenpeace has called it a 'monument to the binge-flying culture'.

Opponents include local people and even London Mayor Ken Livingstone, who are against the plans on pollution and congestion grounds.

But Mr Rudd defended the expansion, saying: 'We have to compete in the world. The nation has to decide whether we want to be a world class nation or a second-class nation.'

REUTERS, ASSOCIATED PRESS, AGENCE FRANCE-PRESS