PARIS (Reuters) -There are advantages of new entrants joining an Italian-British-Japanese next-generation fighter jet programme, but it's a decision for the governments involved, the head of Italian aerospace and defence group Leonardo said on Tuesday.

Speaking at the Paris Airshow, Roberto Cingolani said more partners could help boost the technology of the Global Combat Air Programme.

He also said that Saudi Arabia joining the programme - which has long been mooted - would open up a big market with great potential.

Cingolani said he saw room for many partners in future, while reiterating that this would be a political decision.

He added that the project's 2035 deadline was "serious and strict" and "we must make all efforts to meet it".

"We are on schedule but it is a complex development," he added.

Two sources close to the matter told Reuters last month that Japan had growing doubts that the project would meet the 2035 target, potentially forcing Tokyo to plug air defence gaps with new U.S. F-35 stealth planes or upgrades to aging jets.

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 in Japan, BAE Systems in Britain and Leonardo in Italy.

France, Germany and Spain are working on a rival project, although one of its partners - Airbus - said in January it could see the two programmes combining, or at least designed to work together.

(Reporting by Giulia Segreti. Editing by Mark Potter)