By Mauro Orru

The U.K.'s BAE Systems, Italy's Leonardo and Japan's Mitsubishi Heavy Industries secured a contract valued at 4.6 billion pounds ($6.14 billion) to develop a new stealth fighter jet, a boost for the three nations as they seek to bolster their security and defense capabilities.

The Global Combat Air Program, or GCAP, aims to deliver by 2035 a new aircraft equipped with cutting-edge technologies powerful enough to fend off potential threats over the next few decades. The U.K., Italy and Japan established an international agency to oversee the program, while a joint venture of BAE Systems, Leonardo and Japan Aircraft Industrial Enhancement Co.--controlled by Mitsubishi Heavy Industries--is responsible for the design and development of the aircraft.

Edgewing, the trilateral joint venture in which the three partners command stakes of roughly 33.3%, said Friday that the 18-month contract from the international agency would allow it to complete the advanced concept and assessment phase of the program and move ahead with detailed design and development of the fighter jet. The three nations are jointly funding the contract.

"The program is vital for global security and defeating future threats, while sharing costs, technological advantages and creating highly skilled jobs in all three nations. With this long-term funding, the future of GCAP has never been more assured," Masami Oka, chief executive of the GCAP Agency, said.

The contract award is a win for the security partnership among U.K., Italy and Japan, enabling their respective contractors to move forward with design and development plans. GCAP's progress stands in stark contrast with a rival aircraft program that was bedeviled by disagreements between the companies involved.

The Future Combat Air System program, or FCAS, which included Airbus, Dassault Aviation and Indra Sistemas--aimed to build by 2040 a sixth-generation manned fighter jet kitted with artificial intelligence, advanced stealth capabilities, and connected to a swarm of drones and a combat cloud.

However, a spat between Airbus and Dassault Aviation over which should take the lead on its development was never resolved and the program to replace Rafale jets in France's fleet and Eurofighter Typhoons in Germany and Spain's fleets stalled. Germany pulled out of the project with France and Spain last month.

Write to Mauro Orru at mauro.orru@wsj.com