Germany's defence minister on Monday resisted mounting pressure to resign over a scrapped spy drone deal, as Chancellor Angela Merkel said she continued to back him.

Thomas de Maiziere, a close Merkel ally, faced reporters for the second time in a week just three months before a general election to dismiss calls for his head over millions in wasted taxpayer money.

De Maiziere reiterated that he had made mistakes in overseeing the so-called Euro Hawk drone project but had taken the right decision to pull the plug on it last month.

"The end result, which came about via a faulty process, is for me no reason to resign but rather grounds and an impetus to avoid such mistakes in future," he said.

Yet in a further bid to ratchet up the pressure, the centre-left opposition said immediately after the minister's news conference that it would open a parliamentary inquiry into the so-called Euro Hawk affair before the September 22 poll.

It has stepped up calls in recent days for De Maiziere to take his leave, arguing that he misled deputies in testimony on the scandal.

However Merkel's spokesman Steffen Seibert stressed again Monday that De Maiziere had her "full confidence".

De Maiziere has acknowledged that he was kept in the dark for too long on problems with the unmanned surveillance aircraft programme, which had already gobbled up more than 500 million euros ($660 million) before he axed it.

Officials feared aviation authorities would not certify the Euro Hawk -- a version of US-based Northrop Grumman's Global Hawk customised by Europe's EADS -- because it lacks an anti-collision system.

The ministry decided the cost of adding such a system was too high, in what German media have dubbed the "drone debacle".

De Maiziere had figured on the short list of possible candidates to eventually take the reins from Merkel.

But the drone scandal has largely silenced such talk.

Merkel has a wide lead in opinion polls but the Euro Hawk affair has this month become a time-consuming distraction for her government.