Shiny, spacious and still having the construction dust brushed away before opening next month, Heathrow's Terminal Five is the latest bid to help unclog the world's busiest international airport.

After the longest public inquiry in British planning history, lasting nearly four years, the 4.3 billion pound passenger terminal may go some way to easing the frustrations of the 67.3 million passengers who squeeze through the airport which began as a tented village in 1946.

But even as the shops from Gucci to Prada to Harrods are being fitted out in the terminal, due to open on March 27, protests are mounting at plans for another phase of expansion aimed at enabling Heathrow to keep pace with a forecast doubling of flights in Europe over the next 20 years.

A public consultation on the next steps finishes on Wednesday, with campaigners furious over noise pollution, carbon emissions and local disruption. They also question the validity of studies showing expansion to be vital to the British economy.

Experts say around one-third of Heathrow flights are currently delayed - one of the highest rates in the world. Business leaders have long complained of frustration and wasted time, and some argue the logjam is jeopardising London's financial centre.

"Bankers already hate flying from Heathrow," said Tom Otley, editor-in-chief of magazine Business Traveller UK. British newspapers repeatedly criticise the airport and even airport operator BAA is damning about its current facilities.

"Heathrow is old and tired," said spokesman Simon Baugh. "Terminal Five should allow us to start changing that. The effect should be immediate."

Three of Heathrow's four existing terminals were built before 1968. Anyone who has travelled through the airport has joined crowds from around the globe which throng around ageing baggage carousels in low-ceilinged halls.

Unexpected events such as a 2006 security scare have left passengers waiting for hours or days.

With a capacity of 35 million passengers a year, Terminal Five can handle half the airport's current throughput of passengers, taking pressure off existing terminals and opening scope for their redevelopment.

Ceilings are higher in the new terminal, and vast glass walls offer views across the airport. BAA says the new gates, taxiways and parking spaces also offer more flexibility than anything at present, which should reduce delays.

After six years of building and testing, the new terminal - which will take only British Airways flights - will include a range of restaurants. The champagne bottles are already lined up in the business class lounge.

MORE NEEDED?

But with Heathrow's two runways already stretched to capacity, Terminal Five will offer no new flights, just new terminal capacity. BAA and the government now want space for new flights.

As airlines keep snapping up new planes, they are calling for a third runway and sixth terminal to avoid Britain losing out to airports in mainland Europe, and position it to service emerging key destinations particularly in China and India.

"We certainly wouldn't want to get back to the kind of overstretch we have today," said BAA's Baugh.

The company says it wants to avoid delays that slowed the approval of Terminal Five and allowed problems at the existing airport to get worse.

Activists from Greenpeace breached airport security on Monday, climbing aboard an aircraft and unfurling a banner.

But besides general protests at the risk to the global climate in such a development, about two million people who would be under Heathrow's flight path if expansion went ahead are gearing up for a fight. They threaten court action if the expansion does get the green light.

Part of the problem is space. In contrast to most other European airports that remain underused for their size, Heathrow is running out of room. The new terminal was built within the existing airport boundaries - on the site of a sewage works.

To make the next step up, homes would have to be demolished, including 700 in the nearby village of Sipson.

Many houses in Sipson already display posters and banners opposing the new runway - but some are also becoming dilapidated as owners put off refurbishment until they know their fate.

"You do things like put off putting in a new bathroom," said 62-year-old resident Linda McCutcheon. "The money they are offering won't let us get anywhere else around here. But we're certainly not giving up."

With backing from some local councils and London mayor Ken Livingstone, the campaigners believe they can prompt a rethink. Otherwise, they say noise pollution from flights and new road links will blight thousands of properties across particularly West London.

Some residents and environmental campaigners say they may also resort to more direct action, chaining themselves to buildings and blocking bulldozers.

The protesters - many of whom fought Terminal Five but have now largely accepted it bar complaints about light pollution - also say a further expansion would again risk creating the kind of overcrowding seen in recent years.

"Terminal Five might actually make things easier at Heathrow," said anti-expansion campaign chairman John Stewart. "But this new expansion could make everything worse again."