Heathrow's new Terminal 5 is set to open to the public as British Airways begins operating from the building. ( BBC )
A BA flight from Hong Kong will be the first to arrive at the ?4.3bn facility. The first departure is due at 0620 GMT, when a service leaves for Paris.
Both BA and the airport operator BAA are promising a better passenger experience and fewer delays.
Anti-airport expansion groups hope 500 people will take part in a "flash mob" protest at the terminal at 1100 GMT.
Organisers say they intend to carry out a "peaceful and legal event" at the international arrivals area.
The demonstrators - environmentalists and local residents - are opposed to plans for a third runway and sixth terminal at Heathrow.
BAA said it was "people's right" to protest and the airport owner was not expecting passengers to be disrupted.
Until Wednesday, BA services were spread across three of Heathrow's terminals.
The airline has had to embark on a major exercise overnight to move aircraft and handling equipment to the new terminal.
Initially, BA will operate domestic and European services from Terminal 5, as well as long-haul flights that currently operate from Terminal 1.
The first flight to use Terminal 5 will be BA26 from Hong Kong, with the airline's first woman pilot Captain Lynn Barton at the controls. It is expected to land at 0450GMT on Thursday.
The inaugural departure will be flight 302 to Paris, which is due to leave at 0620GMT.
Both BA and BAA have warned passengers it will take time for the new terminal to "bed down".
Meanwhile, BAA has had to drop controversial plans to fingerprint domestic passengers after the information commissioner expressed concerns about the move.
The airport operator said fingerprinting was needed for border security. Instead it will take photographs while the proposal is discussed with the commissioner's office.
While BAA says the opening of Terminal 5 will not see any more flights operating to and from Heathrow, environmental groups are not convinced.
John Stewart, from anti-expansion group Hacan ClearSkies, said: "The real reason it [Terminal 5] was built was to provide the terminal capacity to allow more flights on the existing runways, which will mean more noise, more pollution and more stress for those living under the flightpaths."
Environmental campaigner Andrea Needham, who will be taking part in the "flash mob" said it would be a "fun event with a very serious purpose".
"Climate change can be beaten, but not by almost doubling the size of the world's biggest international airport," she said.
But BA chief executive Willie Walsh, writing in the airline's staff newspaper, said Terminal 5 would not "solve our problems".
"There continue to be problems at Heathrow with a lack of runway capacity and T5 will do nothing to sort this out," he said.
Once BA has completed its terminal changes, BAA plans to start moving other carriers to Terminals 1, 3 and 4, where they will be grouped according to airline alliances.
Terminal 2 will then be demolished, to pave the way for a new facility - Heathrow East - which BAA hopes will be ready in time for the London Olympics in 2012.