By Sabine Siebold

BERLIN (Reuters) - Delays in adapting the Eurofighter jet for air-to-ground operations could make it impossible for Germany to fulfill its promise to contribute to NATO's rapid reaction force in 2018, a German government report seen by Reuters suggests.

Originally developed for air-to-air combat, Airbus's Eurofighter was only later adapted for bombing, the role in which Germany had planned to use it next year in its contribution to the NATO Response Force.

Any delay would be a blow to the German armed forces, which are in the middle of an investment program designed to strengthen their ability to play a larger role in European security in the face of a newly active Russia.

According to the confidential annex to the German government's April report, the aircraft's ground-bombing abilities will only be tested in September this year, meaning time for completing testing will be very short.

A defense ministry spokesperson said the air force was still on track for the deployment. "The air-to-ground capability of the air force will be tested by NATO in the second half of the year," the spokesperson said.

Germany has already made heavy use of its Eurofighter fleet in airspace defense roles in Germany and above the Baltic region, where they are contributing to NATO patrols of a region that is increasingly subject to Russian probing.

Germany's existing Eurofighter fleet had clocked up 76,000 hours' flight time by the end of last year, the report said, noting that improvements were needed to the jets' self-defense, night vision and communications capabilities.

Furthermore, the oldest of the jets, delivered in three tranches, are reaching their limits, the report said. "Tranche 1 is worn out," the report said.

It highlighted other shortcomings, including a shortage of computing power that made it impossible to install the latest radars, while munitions for the aircraft, including air-to-air missiles and precision bombs, were hard to obtain.

"If it came to a deployment, stocks would rapidly be exhausted," the report said.

Germany has ordered 143 Eurofighters, of which 125 had been delivered at the end of February. It is due to be equipped with MBDA's Meteor missiles and BGU-48 laser-guided bombs from Raytheon, which may later be replaced by Boeing's GBU-54 bombs.

(Reporting by Sabine Siebold; Writing by Thomas Escritt; Editing by Catherine Evans)