Heathrow Airport's giant new terminal five opens to passengers on Thursday, with security forces on high alert after threats of eco-protests.

Green militants opposed to expanding the world's busiest international air gateway fear T5 is just the start of a vast new expansion that will see a third runway, sixth terminal and up to twice as many passengers.

The new terminal at the airport, which already squeezes through 67.3 million passengers a year, was formally opened earlier this month by the Queen.

Hundreds of demonstrators are expected to stage peaceful "flash mob" protests at 11 a.m., activists said. They said they would not disrupt flights.

"The Flash Mob is a fun event but with a very serious purpose," said their spokeswoman Andrea Needham.

Despite the peaceful assurances, authorities will be on heightened alert after a series of embarrassing security breaches at Heathrow.

"We will have a robust policing plan in place on the day so that anyone breaking the law is dealt with," Scotland Yard said.

A spokesman for the airport's operator BAA said extra security will be in place.

"We have always said that people have got the right to voice their own opinion, but we don't think any form of protest inside the airport perimeter is a responsible thing to do," he said.

On the eve of the Queen's visit, police charged Ketheeswaran Uthayakumar, 27, who had run out onto one of Heathrow's runways, with "aircraft endangerment" and being illegally airside.

In February, protesters broke into the airport and wrapped a banner around a plane.

The open-plan, 4.3 billion-pound terminal, Britain's largest enclosed space, is to be the new home of British Airways, which will channel passengers past Gucci and Prada boutiques and a restaurant owned by celebrity chef Gordon Ramsay.

BAA says passengers want luxury shops but critics say the company should do more to speed passengers quickly onto planes rather than trying to make money from them as they wait.

The first passengers are expected to arrive from Hong Kong at 4:50 a.m. on Thursday, while the first outbound plane is due to leave the airport at 6:20 a.m. bound for Paris.