Britain will work to develop next-generation fighter jets with Italy and Japan, Rishi Sunak has announced.
The Prime Minister said the defence partnership will ensure the UK and allies are "outpacing and out-manoeuvring those who seek to do us harm".
Downing Street aims for the jets, called Tempest in the UK, to take to the skies by 2035 and serve as a successor to the RAF Typhoon.
The ambition is for the planes, developed under the global combat air programme (GCAP), to be enhanced by capabilities including uncrewed aircraft, advanced sensors and cutting-edge weapons.
They are being developed in response to military bosses' fears that air dominance is being threatened.
Mr Sunak will launch the first major phase of the programme during a visit to RAF Coningsby, in Lincolnshire, on Friday.
Ahead of the visit, he said: "The security of the United Kingdom, both today and for future generations, will always be of paramount importance to this Government.
"That's why we need to stay at the cutting-edge of advancements in defence technology – outpacing and out-manoeuvring those who seek to do us harm.
"The international partnership we have announced today with Italy and Japan aims to do just that, underlining that the security of the Euro-Atlantic and Indo-Pacific regions are indivisible.
"The next-generation of combat aircraft we design will protect us and our allies around the world by harnessing the strength of our world-beating defence industry – creating jobs while saving lives."
The jet is expected to be able to fly faster than the speed of sound, and have the capability of firing hypersonic weapons in the future.
Working with the allies is hoped to share the costs and ensure the RAF can easily work with its closest partners, with the new Tempests being compatible with other Nato partners' jets.
Defence Secretary Ben Wallace said: "This international partnership with Italy and Japan to create and design the next-generation of combat aircraft, represents the best collaboration of cutting edge defence technology and expertise shared across our nations, providing highly skilled jobs across the sector and long-term security for Britain and our allies."
The partnership merges the UK and Italy's future combat air system (FCAS) projects with the Japanese F-X programme.
Ministers hope that other countries may buy into GCAP in due course.
John Healey, Labour's shadow defence secretary, said his party backed the partnership.
"Ministers must make clear how this fits with wider plans for the RAF's future, including how they will prevent delays in fast-jet pilot training and how many F-35 fighters they plan to purchase," he said.