* Turkish Airlines heavily reliant on transit passengers
* Istanbul hopes major new airport will be global hub
* Ban seen as bid to undermine Turkish, Gulf carriers
By Ceyda Caglayan
ISTANBUL, March 23 (Reuters) - As Istanbul builds a massive new airport and Turkish Airlines pursues aggressive route expansion, some in Turkey see the ban on large carry-on electronics on U.S. and Britain-bound flights as a ploy to undermine their aviation industry.
The U.S. and British decisions to forbid devices bigger than a cellphone in the cabin on flights from several Middle East and North African states could hit Istanbul particularly hard, after years building up its position as an international hub.
More than half of international passengers at Turkish Airlines, which is 49 percent state-owned and serves 247 destinations outside Turkey, are transit customers who could instead travel through Europe or other hubs.
Fast-growing Gulf airlines such as Emirates, Etihad and Qatar Airways, which similarly depend on business-class flyers stopping over in places like Dubai or Doha for far-flung destinations, could also be hit.
For Turkey's national carrier, the setback comes as it prepares to relocate to a 10 billion euro ($10.8 billion) third airport in Istanbul, billed by the developer as the world's largest and championed by President Tayyip Erdogan as a megaproject that will cement Turkey's global standing.
"The biggest aim of the third airport was to use Istanbul as a hub. But if such security measures become permanent, this could reduce Istanbul's appeal," said Esra Sirinel, a deputy research director at Is Investment and an airline specialist.
Istanbul's existing main Ataturk airport is struggling to cope with millions of transit passengers a month. The new airport will initially have the capacity for 90 million passengers when it opens in 2018, but ultimately aims to serve double that and to offer flights to more than 350 destinations.
Turkey's pro-government media, which, like Erdogan, often casts the country as a victim of foreign conspiracies, was quick to lambaste the new regulations.
"They're afraid of THY (Turkish Airlines)," the Yeni Safak newspaper said in a front-page headline on Thursday, over a simulated image of the new airport glistening in the sun. It cast the move as a plot against Turkey's national carrier.
"We need to stop THY," read the headline on fellow pro-government daily Aksam, which claimed U.S. carriers, facing stiff competition from Turkish Airlines and Gulf carriers, had lobbied for the move.
Gulf airlines have been battling against a campaign in Washington by U.S. carriers which accuse them of receiving unfair subsidies, charges they deny.
The administration of U.S. President Donald Trump has so far placed more emphasis on the job benefits of building jets for foreign carriers. Gulf carriers and Turkish Airlines are major customers of Boeing and between them have more than 440 Boeing jets on order, worth tens of billions of dollars.
The U.S. restrictions apply to flights originating from 10 airports in countries including the United Arab Emirates, Qatar and Turkey. The British restrictions do not include the UAE or Qatar but will affect Turkish Airlines and UK-based carriers including British Airways, easyJet and Monarch.
A senior Turkish government official questioned the basis for the restrictions, saying airport security in Turkey was no worse than in Europe and that laptops or tablets could still be used to carry explosives even if they are put in the hold.
"It appears this decision was made purely on commercial grounds, and it seems it was done to limit the advancements made by Turkish Airlines and by Turkey," the official told Reuters.
Security experts say Washington and London may believe a laptop could disguise a bomb big enough to bring down a plane, warranting extra explosives checks which can be performed on luggage in the hold. Confining any device to checked baggage would also make it harder for a would-be attacker to trigger it, or to position it to bring the plane down.
Turkey's foreign ministry said talks were underway to try to persuade the United States and Britain to exclude Turkish Airlines and Istanbul airport from the ban.
Istanbul-based brokerage Unlu&Co said Turkish Airlines and Gulf operators would be hit hardest by the regulations.
"(They) have the most to lose from the U.S. ban since they rely on transfer passengers who may need access to laptops and other devices for business reasons and could easily travel via European hubs, like Frankfurt," it said in a report.
Emirates said it would introduce a service letting customers use laptops and tablets until just before they board direct flights to the U.S. from Dubai, and made clear on Thursday that the ban did not apply to its U.S.-bound flights via Europe.
The Americas account for 14 percent of Turkish Airlines revenue, according to the airline's website, which does not break out figures separately for the United States or Britain.
Turkish Airlines did not immediately respond to requests for comment.
$1 = 0.9270 euros Additional reporting by Alexander Cornwell in Dubai, Alana Wise in New York, Tim Hepher in Paris; Writing by Tuvan Gumrukcu and Ece Toksabay; Editing by Nick Tattersall and Peter Graff