FARNBOROUGH, England, July 24 (Reuters) - A UK

initiative to promote green jet fuel will need to be taken up

around the world - and built on - if the technology is to have

any chance of delivering its promise of radically cleaner

flying, airline executives warned at the Farnborough Airshow.

Last week, Britain's new Labour government announced plans

to introduce a price guarantee for sustainable aviation fuel

(SAF) to incentivise producers to open more plants and build

infrastructure to ramp up the fuel's production.

Two decades after airlines pledged to switch to biofuels,

SAF accounts for just 0.2% of the jet fuel market. The aviation

industry says this will need to rise to 65% by 2050 if it is to

reach "net zero" carbon emissions by then.

Airlines and SAF producers have been caught in a paralysing

blame game for years. Airlines say they want more green fuel,

while SAF producers say they can't make more until airlines

agree to pay the market price. SAF currently costs up to five

times more than traditional jet fuel.

Julie Kitcher, chief sustainability officer at Airbus

, said more solid investment plans and bolder financing

from across the sector would help bring the industry to scale

and boost supply.

Policy support, such as the UK's mechanism or Singapore's

SAF levy, can take the cost burden off airlines, if crafted

constructively, Kitcher and other executives said.

"It's really about back to basics ... it's about going

through the value chain of the whole aviation system and

everybody taking a share, because we won't get there without

it," she said.

Still, airlines lament a shortage of SAF supply despite

mandates and incentives, arguing that producers, particularly

highly-profitable oil companies, aren't making enough.

"If we don't reduce the price of SAF, flying is going to be

much more expensive," said Luis Gallego, chief executive of

British Airways-owner IAG, during a panel discussion at

the air show after praising the UK's decision.

Some have warned that airlines could miss targets to use 10%

of the fuel to power their planes by 2030.

At the air show, airlines and manufacturers pointed to the

next generation of more energy-efficient aircraft as a way to

reduce emissions.

But many environmental advocates say the growth of the

industry is fundamentally incompatible with sustainability.

Airbus, the world's largest planemaker, expects the global

aircraft fleet will more than double over the next two decades

to 48,230 planes.

"If the amount of planes in the sky goes up, emissions will

only keep going up more and more even if the planes are

marginally more efficient," said Matt Finch, UK policy manager

of advocacy group Transport and Environment.

"Every single plane that's ordered can only burn

hydrocarbons, and there just aren't the SAF volumes coming on to

match the plane orders."

STILL NOT ENOUGH

Even with revenue security, much more investment would be

needed for the aviation sector to reach its goals, Virgin

Atlantic Chief Executive Shai Weiss told Reuters.

"That (mechanism) alone is not enough," he said.

Earlier this month, Shell announced it was pausing

construction of a SAF facility in Rotterdam, alarming green

advocates.

Even with stronger government support, new plants would

still take years to build, let alone start producing ample

supplies of SAF.

Airline executives said the UK move was a step in the right

direction, but that it was legislators who needed to do more,

not airlines.

"We know that decarbonisation is going to be very expensive.

It's going to cost trillions," IAG's Gallego said.

Executives point to thin profit margins in the aviation

sector, allowing little room for investment in sustainability.

"This is where the government has to step up the production

of the availability of fuel for us to be able to use," said

Ghaith al-Ghaith, chief executive of flydubai.

"The airline burden is to have a successful operation, safe

and secure."

(Additional reporting by Allison Lampert, Tim Hepher, Joe

Brock, David Shepardson; Editing by Mark Potter)