There has been mounting speculation that the founding countries in the Global Combat Air Programme could open the door to Germany after a rival Franco-German fighter programme collapsed in June, while Saudi Arabia has long been touted as another potential partner.
Leonardo is working on developing the new jet with BAE Systems (BAES.L) and Mitsubishi Heavy Industries (7011.T).
Mariani stressed that while the decision was one for the three governments, the clock was ticking from an industry perspective, given the 2035 target for GCAP to be flying.
"My personal opinion 2026...maximum one year from now," he said on Wednesday when asked in an interview at the Farnborough Airshow about a deadline for countries joining.
Mariani said Germany would be a suitable partner given its expertise, its growing military budget and the fact that it already works on the Eurofighter Typhoon with the UK and Italy, but some executives have warned that adding new partners could lead to delays.
"I think, I believe, there is still an interest of Saudi to be part of this programme," he added.
GCAP has been set up to allow differing levels of involvement and it expanded for the first time on Tuesday, when Canada signed up as an observer, which could pave the way for deeper engagement in future.
Mariani said the 2035 timeline could even be brought forward.
"Now there has been a request, and let's say push, to even anticipate that date, so this is priority number one," he said.
