PARIS — The French prime minister has rejected a proposal by the government’s accounting watchdog that future Ariane rocket development be conducted outside the European Space Agency (ESA) to cut costs and consolidate France’s role as the rocket’s designer and operator.

In a letter to France’s Cour des Comptes, which is the French equivalent to the U.S. Government Accountability Office, Prime Minister Jean-Marc Ayrault said removing ESA from future Ariane work would exacerbate France’s difficulties in getting other European governments and institutions to help finance Europe’s Guiana Space Center spaceport in French Guiana.

Ayrault said any hope France has of getting spaceport financial support from the European Commission likely would be dashed if future Ariane work was conducted by just a handful of nations, including France.

Beyond the financial costs, Ayrault says, the political costs would be equally heavy.

“It would represent a step backward for Europe’s space sector and for the European project as a whole, of which Ariane is a symbol,” Ayrault said in response to the accounting court’s recommendations.

Any move to consolidate Ariane work in France would also encourage other European governments to launch their satellites elsewhere, he said.

Ayrault nonetheless appeared to agree with the accounting body’s proposal that the French government take steps to assure that French industry alone is capable of maintaining future Ariane development and thus of assuring independent access to space.

Ayrault’s letter to the accounting court, dated April 3, was published on the court’s website. French government finance law requires such a response within two months of a report on a given government investment. The letter followed an accounting court analysis of France space transport policy that was submitted to the office of the prime minister, and the French defense and research ministers, Jan. 28 and made public with Ayrault’s response.

The accounting court’s report said that since France is the main financier of the Ariane program, it should eliminate the costs associated with working within ESA, and the costs associated with spreading work among many ESA nations, by assuming future Ariane development with just a couple of partners such as Germany and Italy.

The court said other ESA governments complain about having to contribute their pro rata share each year to help cover the operating-cost losses from the Arianespace launch consortium, while reaping the benefits of having their national industries contributing to the success of Ariane 5 in the commercial market.

The well-known cost of having Ariane produced in sites across Europe could be reduced if future rockets had a smaller industrial base, making the net cost to French taxpayers marginal, the court said. That such a proposal could be made by an independent, finance-minded body suggests how fragile Europe’s launch vehicle consensus is despite the commercial success of the Ariane rocket line.

But it is not just Ariane. The court also is wary of Europe’s use of Russia’s Soyuz medium-lift rocket from the European spaceport. The Euro-Russian agreement on Soyuz continues only to 2016, but it will take until well after 2020 before the debt incurred to finance the project is fully repaid.

Evry, France-based Arianespace secured a French government guarantee for a low-interest loan of 121 million euros ($157 million) from the European Investment Bank.

The accounting court said that even if Russia agrees to continue the Soyuz agreement beyond 2016, the arrival in 2021 of the Ariane 6 rocket, now being studied by ESA, could compromise the loan insofar as Ariane 6 would compete with Soyuz for European government business.

The report raises separate concerns about Europe’s new Vega small-satellite launcher, whose development is led by Italy. It says Vega is too small to take over from Soyuz in launching European government satellites, and too expensive to win global market share from competing vehicles.

In his response, Ayrault said Arianespace has already repaid 45 million euros of the European Investment Bank loan and that most, if not all, of the outstanding balance will be paid by 2021. Even if some balance remains, he said, it will be small compared to the investment France will make in Ariane 6 and thus will not be an issue for Ariane 6-Soyuz competition.

On Vega, Ayrault said the accounting court has assumed heavy competition from converted Russian and Ukrainian ballistic missiles whose usability will be running out in the coming years. Ayrault acknowledged that Vega’s production costs must be reduced. ESA has purchased the first five Vega rockets to permit Vega builders to gradually streamline the vehicle’s production and trim production and operating costs.

Ayrault faults the court’s analysis of the Guiana Space Center, saying it is not fair to include the cost of French military units that would be there even without a space program. Finally, he said, the court omitted in its report the fact that investment by ESA governments in the spaceport stimulates French Guiana’s entire economy, providing benefits to the mainly French businesses that do not work in the space sector.

Peter B. de Selding was the Paris Bureau Chief for SpaceNews.