By Katy Austin, Transport correspondent, Nov 28: The first transatlantic flight by a large passenger plane powered only by alternative fuels will soon take off.
Operated by Virgin Atlantic, it is expected to fly from London's Heathrow to New York's JFK airport at 11:52 GMT.
Airlines see the flight, which is supported by government funding, as demonstrating that a greener way of flying is possible.
But a lack of fuel supply remains a challenge, while other technology will be needed to hit emissions targets.
The flight is a one-off of its kind so far and is not carrying fare-paying passengers.
So-called sustainable aviation fuels (SAF) can be made from a variety of sources, including crops, household waste and cooking oils.
For this flight, a Boeing 787 will be filled with 50 tonnes of SAF. Two types are being used, with 88% derived from waste fats and the rest from the wastes of corn production in the US.
Following test and analysis, the flight was approved by the UK regulator the Civil Aviation Authority earlier this month. A number of companies have been involved in the project including engine maker Rolls-Royce and energy giant BP.
The aviation industry is particularly difficult to decarbonise, but airline bosses view SAF as the most effective tool available to help bring its net emissions down to zero.
Planes still emit carbon when using SAF, but the industry says the "lifecycle emissions" of these fuels can be up to 70% lower.
Shai Weiss, chief executive of Virgin Atlantic, said the airline's flight on Tuesday was "proving... that fossil-derived fuel can be replaced by sustainable aviation fuel".
"It's really the only pathway to decarbonising long-haul aviation over and above having the youngest fleet in the sky," he told the BBC's Today programme. "It is a really momentous achievement."
However, he said there was not enough SAF currently, and added that due to the fuel being more expensive, flight prices would end up being higher.
Virgin founder Sir Richard Branson admitted it was "going to take a while" before there was enough SAF for everybody to use.
"But you have to start somewhere," he told the BBC. "And if we didn't prove it can be done, you would never, ever get sustainable aviation fuel."
SAF is already used in small amounts, blended with traditional jet fuel, but accounts for less than 0.1% of the aviation fuel consumed around the world.
It currently costs more than kerosene, and relatively small amounts are made. Aircraft are usually only allowed to use up to 50% in a blend.
There are no dedicated commercial SAF plants in the UK, although the government's aim is to have five under construction by 2025, supported by grant funding.
Airlines see the first long-haul flight of a large passenger plane using 100% SAF as a significant milestone. But experts say such fuels are not a magic bullet.
Dr Guy Gratton, associate professor of aviation and the environment at Cranfield University, said: "We can't produce a majority of our fuel requirements this way because we just don't have the feedstocks. And even if you do, these fuels are not true 'net zeros'."
He said the growing use of SAF had to be treated as "a stepping stone towards future, genuinely net-zero technologies".
"This might be e-fuels [which are manufactured using captured carbon dioxide or carbon monoxide, together with hydrogen], it might be hydrogen, it might be some technologies that we still really only have at the laboratory stage."
The policy director of the Aviation Environment Federation campaign group, Cait Hewitt, said the idea that the flight meant the UK was closer to "guilt-free" flying was "a joke".
She said there might be better technology in the future to cut carbon emissions but, for now, the only way to achieve this is to "fly less".
UK ministers and the industry have insisted they believe "net zero" by 2050 is achievable with passenger numbers increasing.
Transport Secretary Mark Harper told BBC Breakfast: "There are those campaigners who want to tell ordinary people that they can't fly. That's their view, they're entitled to it. The government doesn't agree with them.
He said using SAF produced about 70% less carbon emissions than traditional fuels "so that is a really big step forward".
"We are also involved with supporting the industry to develop hydrogen and also electric flights for shorter-haul flights, so all of that technology is being developed."
Harper acknowledged that using SAF was "not the only solution", but said: "It is a really important step with those other technologies to make sure we can carry on flying and protect the environment."
The UK government plans to require 10% of aviation fuel to be SAF by 2030.
Airlines UK, which represents UK-registered carriers, said they must be able to access enough affordable SAF to meet such a requirement, with as much as possible coming from the UK.
Its boss Tim Alderslade said: "The last thing we want is higher fuel costs for UK passengers compared to the rest of Europe and the US, with worse sustainability outcomes and thousands of new jobs lost overseas."