WASHINGTON — The U.S. company that markets Russia’s Proton rocket will submit a bid this spring to the European Commission to launch 18 Galileo navigation satellites on three Proton rockets in what the firm says may be the commission’s only hope for staying within its Galileo budget.

Reston, Va.-based International Launch Services (ILS) is prepared to tell European authorities that three Proton launches carrying six Galileo satellites each, combined with the five Soyuz rockets already purchased to launch Galileo satellites two at a time, would allow Galileo managers to stay within their launch budget of 700 million euros ($1 billion).

The European Commission in January contracted for five Europeanized versions of Russia’s Soyuz rocket for 397 million euros — far higher than what had been budgeted. The higher-than-expected launch charge is one factor that is forcing the Galileo program to slow the purchase of satellites.

Fourteen satellites have been ordered for a constellation that is supposed to be 30 satellites.

“We can help them close the business case,” ILS President Frank McKenna said March 16 during the Satellite 2010 conference in Washington. “We have taken a close look at this, and a mix of the Soyuz vehicles and the Proton will come in beneath the 700 million-euro ceiling.”

McKenna said the ILS bid should not be viewed as a response to a formal request for proposals from the commission. Galileo project officials have long said they would not consider launches from outside European Union territory. The European version of Russia’s Soyuz will be launched from Europe’s Guiana Space Center in French Guiana.

McKenna said the commission has encouraged ILS to submit a bid, but provided no guarantees as to whether it would be accepted.

Galileo program managers have given no indication that they are willing to consider vehicles other than the Europeanized Soyuz and Europe’s own Ariane 5 heavy-lift rocket, which can carry four Galileo spacecraft at a time. But with no fresh financing in sight, they are under pressure to find unorthodox solutions to put the full constellation into service.

Peter B. de Selding was the Paris bureau chief for SpaceNews.