The Queen has officially opened Heathrow Airport's controversial Terminal 5, describing it as "a 21st Century gateway to Britain".
Hundreds of airport and construction workers attended the opening ceremony.
The £4.3bn terminal, set to begin operating on 27 March, will offer extra passenger capacity but no more flights.
Operator BAA said it would put Heathrow at "the leading edge of global travel" but environmental groups say it will lead to more flights and pollution.
'Advert for Britain'
The Queen, who was accompanied on the visit by the Duke of Edinburgh, addressed some 800 invited guests, many of whom were involved in the project.
She spoke of the "bright, airy space and clean, efficient layout", as she declared the terminal ready for business.
"It gives me great pleasure to open Terminal 5 - this 21st Century gateway to Britain and, to us, the wider world," she said.
The royal couple, dignitaries and hundreds of workers heard a specially-commissioned piece of music performed by a 30-strong choir.
Speaking at the opening ceremony, Sir Nigel Rudd, chairman of airports operator BAA, which has funded the terminal, said the building represented "a living, breathing advertisement for Britain's ambition".
"Terminal 5 marks the start of a new beginning for Heathrow, for BAA and for our millions of passengers," he said.
"It is by any standard a triumph of ambition, commitment and collaboration. It will breathe new life into Heathrow, allowing us to continue our transformation of the rest of the airport and will put Heathrow and BAA back where they belong - at the leading edge of global travel."
Transport secretary Ruth Kelly said it was "destined to become one of London's most iconic transport buildings".
She praised its stunning design and spectacular scale, and said it was one of the greenest airport buildings in the world.
Ahead of the ceremony, the Queen met architect Lord Rogers and the BA crew of the Boeing 777 who averted a tragedy when they crash-landed short of the runway at Heathrow in January.
The opening follows a major security alert at the airport on Thursday after a man with a rucksack scaled the perimeter fence and ran into the path of an aircraft.
Ketheeswaran Uthayakumar, of no fixed abode, has been charged with endangering aircraft security in relation to the incident and is due before Uxbridge magistrates in west London on Saturday.
Some 60,000 people have worked a total of 100 million man hours to build Terminal 5 since construction began in September 2002.
Built on the site of a former sewage works at the western end of the existing airport, Terminal 5 has been designed by 2006 Stirling Prize winners Rogers Stirk Harbour and Partners.
Its construction has involved diverting two rivers, building what is claimed to be the UK's largest free-standing building and tunnelling 13km for rail and baggage links.
The complex includes 50 new aircraft stands, which will rise to 60 by 2010, two satellite buildings, one of which is still to be completed, rail links to London Underground and the Heathrow Express, and a new multi-storey car park.
The official opening is of the project's Phase 1, including Terminals 5A and 5B. Phase 2, which adds Terminal 5C, is set to open in 2010.
BAA says it will simplify the process of checking in for flights with online check-in, fast baggage dropping facilities and state-of-the-art baggage handling.
The first passengers will start using Terminal 5 in a fortnight when they step off a BA flight from Hong Kong.
The terminal will be solely for the use of British Airways for international flights and journeys to and from Manchester, Newcastle, Belfast and Scottish airports.
BA chief executive Willie Walsh said it was not unfair for the airline to have sole access to the terminal because for the first time in 40 years, it could bring most of its operations under one roof.
HAVE YOUR SAY
It's better to concentrate the traffic effectively at Heathrow, rather than building yet another airport
Lee Rixon, UK
Friends of the Earth's aviation campaigner Richard Dyer said if the government was serious about tackling climate change, the opening of Terminal 5 would mark the end of airport expansion in Britain.
"The government must abandon its plans for a third runway and Terminal 6 - it should invest in fast rail links and ensure that Britain's share of international aviation emissions is included in its new climate change law," he added.
John Stewart, of residents' campaign group Hacan Clear Skies, said the opening marked a "sad day" for people living near Heathrow.
He accused successive governments of going back on promises that there would not be a fifth terminal at the airport, saying: "The Queen will be unveiling another broken promise rather than a sparkling new terminal.
"The pall of deceit and collusion will hang over the entire proceedings."
The Queen opened the airport's first terminal building - which is now Terminal 2 - in 1955.