PARIS — Airbus group and Safran have signed an agreement to put strategic missile launchers into their 50/50 space launchers joint venture, the two companies said in a joint statement.

"Both companies will contribute to the current joint program with industrial assets dealing with civil space launchers and military launchers," the companies said.

Airbus Defence & Space is prime contractor for the M51 new generation strategic submarine-launched ballistic missile, and was notified in 2014 of a contract for the M51.3 upgrade. The French Navy is upgrading its four Triomphant class nuclear missile submarines with the M51, which entered service in 2010.

Signing the agreement is the second and final stage of the Airbus Safran Launchers joint venture, the companies said.

The 50-50 ownership reflected a rare case of "strategic shareholdings" in a "strategic structure," Marwan Lahoud, Airbus chief strategy and marketing officer and chairman of Gifas, the French aerospace trade body, told journalists May 3. Only European missile maker MBDA has a similar ownership structure.

Safran has agreed to pay €800 million (US $920 million) to Airbus to acquire an equal holding in the space launcher joint venture, an industry executive said. The value of Airbus’ space business, including upgrading M51, would have given the aerospace group a 70 percent stake, leaving Safran with 30 percent.

That imbalance led to Safran agreeing to pay the €800 million to bring in the 20 percent to allow an equal ownership of the joint venture. Safran brings its Herakles subsidiary, a solid rocket motor specialist, into the deal.

Payment of €800 million for a 20 percent stake implies a valuation of €4 billion for the planned joint venture.

The new company is expected to play a key role in building the planned Ariane 6 rocket, seen as key to replying to competition from the Space X, a US privately owned launcher, Reuters reported.

A first phase led to Airbus and Safran putting their civil space and launcher contracts into a joint program company.

"In this second and final phase, industrial assets and military launchers will be integrated in the joint venture," the companies said. "Airbus Safran Launchers will be a fully fledged operational company."

The joint venture agreement is expected to close by the end of June, the companies said.

France is studying a successor to the Triomphant boat, which will carry ballistic nuclear missiles based on the M51 but not a new generation weapon, according to "The President and the Bomb" (Odile Jacob), a book co-authored by Jean Guisnel and Bruno Tertrais. "There will not be an M6, " says the book, published last week.

Work has begun on the M51.3 version, intended to enter service around 2025, the book says. The aim is for the weapon, tipped with the

tête nucléare océanique

or oceanic nuclear warhead, to defeat all missile defense systems.