LONDON, Sept 13 (Reuters) - Italy was committed to fund its share of the Italian-Anglo-Japanese fighter jet programme for 15 years, long enough for the aircraft to enter service, the director for the project at the Italian Ministry of Defence said on Wednesday.

The next-generation fighter project, named GCAP, is expected to cost tens of billions of dollars, but the three partners have not yet finalised how the budget, or the work will be split.

The lead contractors - Britain's BAE Systems (BAES.L), Japan's Mitsubishi Heavy Industries (7011.T), and Italy's Leonardo (LDOF.MI) - said on Tuesday they had agreed the next steps to deliver the concept phase of the aircraft.

The three, along with their respective defence ministries, appeared together at London's DSEI arms fair to discuss the project, which aims to produce the fighter jet by 2035.

Luca De Martinis, Fourth Department Director at the Italian Ministry of Defence, said there was "full commitment to sustain the programme" from the department and from politicians, with a line of finance in the defence budget of "15 years length".

Reuters, citing two sources, said in March that Britain and Japan were set to dominate the project, with Rome set to pay around only a fifth of the overall development cost.

Italy's defence ministry said at the time that the sources' assessments were "speculative", while Britain's Ministry of Defence (MOD) said it "does not recognise these comments".

Richard Berthon, director Future Combat Air at the MOD, said on Wednesday that unlike previous joint projects, the budget for GCAP would not be divided up like a cake between the countries in the early stages.

Instead there would be flexibility as the partners' industrial capabilities evolved through the programme, he said.

Berthon said a number of other countries had shown interest in the project, but the main focus for all three partners was on delivering the aircraft within the challenging time frame.

Asked about interest from Saudi Arabia, which has a longstanding relationship with Britain in air defence, he said there had been conversations to explore future possibilities, but "no decision beyond that".