The agreement, first reported by La Tribune, paves the way for political talks to resume among the project's three government backers, France, Germany and Spain, a source close to the matter told Reuters.
"The governments have received an offer from the companies to build the demonstrator for a new combat plane in the framework of FCAS," the French armed forces ministry said, adding that negotiations were continuing on the overall project.
Rolls already had a formal exclusive agreement to provide Trent XWB-family engines on the larger A350-1000 wide-body jet, and held what industry sources have described as "de facto" exclusivity on the stronger-selling A350-900 until 2025.
The decision effectively postpones any re-engining of the A350 to next decade while giving Rolls more clarity as it tackles the costly UltraFan development, industry sources said.
That improvement depends on airlines flying 55% of 2019 levels during 2021. Rolls-Royce said its assumption is for travel to gradually improve this year, accelerating in the second-half as vaccine programmes progress.