KARLSRUHE - Germany's highest court began deliberations on Tuesday on a request by opposition parties to block the purchase of 73 Airbus A400M military transport planes, which has cast new doubts on the eight-nation deal.

Germany, which accounts for the largest number of aircraft in the 196-plane, 18-billion-euro ($15.5 billion) project, has until the end of January to clarify funding or the other seven buyers could pull out from a deal seen as key to closer European military cooperation and a planned rapid reaction force.

The opposition Christian Democrats and Free Democrats appealed to the Constitutional Court last week to grant an injunction to stop the government signing up for 73 planes, saying parliamentary budget procedures had been circumvented and only part of the cost of the project was secured.

"The government should not make promises in foreign policy that it cannot keep in domestic politics," Friedrich Merz, CDU parliamentary leader, said before proceedings started at the court in the central town of Karlsruhe.

A court decision was expected on Tuesday afternoon.

Merz said the conservatives were not against the project but wanted to protect parliament's budgetary prerogative. FDP parliamentary leader Wolfgang Gerhardt agreed that parliament should not be bypassed on such an important decision.

The German parliament approved financing in its 2002 budget for 40 A400M aircraft, worth just more than five billion euros. Last week, it agreed on a parliamentary statement of intent that Germany would include funding for the remaining 33 planes, worth around 3.5 billion euros, when it draws up its 2003 budget.

Keen to embarrass Chancellor Gerhard Schroeder eight months before an election, the opposition says that circumvented parliament and wants the government to reopen the 2002 budget, which was passed in November, to secure the full funding.

The government has rejected this idea as it would result in long-winded budget proceedings and give the conservatives a chance to attack its handling of the slowing economy.

If the court grants an injunction, the government would either be forced to reopen the 2002 budget or ask its European partners to accept its signature of the deal with a proviso that parliament still has to approve the full funding.

Defence Minister Rudolf Scharping, who signed a deal with Germany's European partners in November committing the country to buying 73 planes as long as parliament agreed, declined to comment as the court deliberations got underway.

The government is expected to argue that last week's statement of intent was a political declaration and would not legally bind Germany until parliament backed the 2003 budget.

Hans Georg Wagner, the budget spokesman for the ruling Social Democrats, dismissed the opposition's court bid as "nonsense" and said if the conservatives were in power they would also buy the full 73 aircraft.

"This is all a lot of fuss about nothing. Scharping has explained on behalf of the government that they will only sign when the parliamentary requirements are settled," Wagner told Berlin's Inforadio in an interview.

Other signatories to the contract sealed in December with Airbus Military, controlled by European aerospace giant EADS, are Spain, Turkey, Belgium, Portugal and Luxembourg. Italy dropped out of the project late last year.

Analysts fear that if the A400M project collapses, the deployment of European soldiers by air will be left largely to the British and the French, who could buy existing U.S. aircraft instead, benefiting Boeing Co and Lockheed Martin Corp.