BERLIN (Reuters) - German opposition figures have stepped up pressure on the defense minister, a close ally of Chancellor Angela Merkel, to resign after accusing him of making misleading statements over a scrapped contract for reconnaissance drones.

Thomas de Maiziere, who was once Merkel's chief of staff, said last month that Germany was dropping the purchase of four Euro Hawk drones made by EADS and Northrop Grumman because it would cost too much to ensure they meet required flight safety standards.

Opposition politicians have said the fiasco wasted some 680 million euros ($900 million), accusing de Maiziere of withholding information about the project's pitfalls and failing to pull the plug on it sooner.

According to internal government documents obtained by Reuters, it would have cost around 500-600 million euros to adapt the drones to meet the safety requirements, without any guarantee that approval would be achieved.

The opposition renewed its attacks after German media reported this weekend that de Maiziere had given varying accounts of when he was first informed of problems with the drones. Some in the opposition accused him of lying.

If de Maiziere quit, it would be a blow for Merkel as she prepares to contest a September election for a third term in office.

Other Merkel allies have been forced to step down in recent years, including her choice of president, Christian Wulff, education minister Annette Schavan and defense minister Karl-Theodor zu Guttenberg. Another resignation could expose her to further attacks over her judgment.

The Defense Ministry issued a statement denying de Maiziere had contradicted himself.

In an interview with Focus magazine, de Maiziere said: "I definitely knew something of the general problems related to the Euro Hawk. These problems looked like they could be solved. The key thing here is when I was told that the problems could not be solved, and that was on May 13."

Thomas Oppermann, parliamentary floor leader of the Social Democrats (SPD), told the newspaper Bild am Sonntag that de Maiziere's resignation was "just a question of time".

"A defense minister who doesn't tell the truth on such a crucial issue cannot remain in office any longer," he said.

Leading Greens politician Juergen Trittin also suggested de Maiziere should go.

"The minister is tying himself up in wild contradictions... He has lost the trust you need to hold this office, so he can't remain minister," he told Bild am Sonntag.

An opinion poll published on Thursday indicated that 83 percent of Germans were unhappy with de Maiziere's handling of the drone affair.

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(Reporting by Andreas Rinke; writing by Michelle Martin; editing by Tom Pfeiffer)