By Tim Kelly and Nobuhiro Kubo

TOKYO, May 30 (Reuters) - Japan has growing doubts that its next-generation fighter project with Britain and Italy will meet a 2035 rollout target, potentially forcing Tokyo to plug air defence gaps with new U.S. F-35 stealth planes or upgrades to aging jets, two sources said.

The joint Global Combat Air Programme $(GCAP)$ established in 2022 is falling behind schedule due to a lack of urgency from Britain and Italy, which could push deployment beyond 2040, according to one of the sources.

Both sources, who have knowledge of Japan's air defence discussions, were not authorised to speak to media and declined to be identified.

Japan will need to begin planning soon if the GCAP is delayed to ensure it has enough jets to counter Chinese and Russian planes that regularly probe its air defences. While senior officials are discussing the possibility of buying more F-35s, Reuters was not able to learn if specific numbers or a timeline are under consideration.

A swift decision to acquire more Lockheed Martin LMT.N F-35s at around $100 million each could be a bargaining chip for Japanese Prime Minister Shigeru Ishiba in tariff talks with U.S. President Donald Trump.

Japan's chief trade negotiator Ryosei Akazawa will hold a fresh round of tariff talks with U.S. officials on Friday in Washington. Ishiba is also expected to meet Trump for their second in-person meeting in June at the Group of Seven leaders' gathering in Canada.

"Japan's purchase of defence equipment would contribute to the U.S. trade surplus, so in that sense, it could be considered (in trade talks)," Akazawa told reporters on Thursday ahead of his departure for the U.S. capital.

In 2019, in a move lauded at the time by Trump, then Prime Minister Shinzo Abe ordered 105 F-35s, making Japan the aircraft's largest overseas buyer with a total of 147 on order.

In a sign that Trump may again welcome Japanese purchases of U.S. military hardware, the Asahi newspaper reported that in a phone call with Ishiba on May 23, he discussed Boeing's BA.N planned F-47 fighter jet and encouraged his Japanese counterpart to consider U.S. aircraft.

GCAP is an effort by Japan and its two European partners to build a next-generation fighter free from the operational restrictions that often come with purchases of U.S.-made military equipment. For example, buyers are often not allowed to do upgrades and maintenance without U.S. permission.

The project is being led by Mitsubishi Heavy Industries 7011.T in Japan, BAE Systems BAES.L in Britain and Leonardo LDOF.MI in Italy.

In Britain and Italy, GCAP will replace Eurofighter Typhoon jets that will be in operation until the 2040s. In Japan, it is meant to replace the F-2, a U.S. F-16 variant developed three decades ago by Mitsubishi Heavy and Lockheed Martin.

Japan's concerns about a possible delay to GCAP's 2035 rollout stem from how much time is being taken to align each country's concept of how the aircraft will operate, said one of the sources.

A spokesperson for the Japanese defence ministry's acquisition agency said the 2035 rollout target was unchanged, adding that the decommissioning of the F-2 is set to begin that year. "I am not aware of any plans to place additional orders for the F-35," he added.

A British Ministry of Defence spokesperson said the three countries were focused on the project and were "making strong progress."

The Italian defence ministry did not reply to a request for comment.

Additional GCAP partners could include Saudi Arabia, which would bring funding and a lucrative market for the aircraft.

Of the 147 F-35s Japan has ordered to date, 43 F-35As have been delivered while six F-35Bs, a short-take off and vertical landing variant, have been delayed, making more purchases a contentious issue.

Japan could instead opt to extend the operational life of its older F-2 fleet through upgrades.

"We have already paid for many items, and they haven't arrived. It's not right to be told to buy more when previous orders haven't been fulfilled," a senior ruling Liberal Democratic Party lawmaker said, speaking on condition of anonymity due to the sensitivity of the issue.

(Reporting by Tim Kelly and Nobuhiro Kubo in Tokyo, and Nathan Layne in New York; Additional reporting by Paul Sandle in London and Angelo Amante in Rome; Editing by Edwina Gibbs)

((tim.kelly@thomsonreuters.com))