Franco-German corporate relations fell to a new low on Wednesday as France’s nuclear champion Areva threatened to sue its German partner Siemens over its decision to set up a rival venture with Russia’s Rosatom.

Areva on Wednesday claimed that Siemens, which is pulling out of its reactor joint venture with the French state-owned group, was in breach of contract, “with all the ensuing consequences” after signing a memorandum of understanding with Rosatom on Tuesday.