FARNBOROUGH, England (Reuters) - A British defence policy review will not derail a multibillion- dollar fighter jet programme being developed by Italy, Japan and the United Kingdom, according to an executive at Leonardo, one of the firms working on the project.

Britain's BAE Systems, Japan's Mitsubishi Heavy Industries ( MHVYF ) and Italy's Leonardo are developing the Global Combat Air Programme (GCAP), which envisions stealth fighters operating in tandem with drones.

The first aircraft are due to enter service in 2035.

British Prime Minister Keir Starmer said on Monday that GCAP was "an important programme", but stopped short of guaranteeing it wouldn't be impacted by the defence review.

There has been speculation that Starmer's new Labour government might downgrade or scrap the programme after it launched the review, which will report next year.

Lorenzo Mariani, co-general manager of Leonardo, said the jet fighter project was too important for the three countries, and European security more broadly, to be cut.

"Normally those processes are quite rapid and quick and conclusive here in the UK so I'm optimistic ... the programme will overcome this review," Mariani told Reuters in an interview.

The United States has said it is reviewing the scale of a similar future fighter platform, known as the Next Generation Air Dominance (NGAD).

France, Germany and Spain are working on their own next-generation fighter project, prompting some in the industry to question whether Europe could sustain two decades-long programmes which could each cost tens of billions of dollars.

Mariani said it was possible if the two programmes had interoperable equipment, including drones, missiles, laser weapons, computing power and data handling.

"The most important thing is to guarantee, that as much as possible, there is commonality in these systems," he said.