The FCAS programme is a flagship joint effort to build a next-generation combat aircraft between France, Germany and Spain, and is seen as a bellwether of European defence and security cooperation.

But the multi-billion-euro project has faltered as disagreements persist between France's Dassault Aviation and Airbus, which represents Germany and Spain.

In a last-ditch bid to rescue it, one mediator from France and one from Germany were tasked last month with coming up with proposals by the end of April.

A German government source confirmed to AFP that the pair had submitted their reports and that Berlin "will discuss them with France in the coming days", without commenting on the outcome.

But according to a report in the Handelsblatt financial daily, the mediators -- the ex-head of a German tankmaker and a former French defence executive -- could not reach an agreement, even producing two different reports.

German Chancellor Friedrich Merz will be briefed on the outcome soon and will then make a decision, before discussing the matter with French President Emmanuel Macron at an EU summit in Cyprus on Thursday and Friday, Handelsblatt said.

Merz, who last month insisted he was determined to find a solution for the project, could still decide to try to push ahead with it.

But calls have been growing even within his own CDU party to pull the plug.

If the mediation fails to produce a workable result, then "FCAS, in its current form, has failed", CDU lawmaker Volker Mayer-Lay told AFP, adding that Germany and France should then focus on developing separate fighter jets.

"The phase of hesitation must not drag on any longer," he said.

The FCAS programme was launched in 2017 to replace the Rafale jet and the Eurofighter planes used by Germany and Spain.