Gabriel, whose Social Democrats (SPD) are junior partner to Chancellor Angela Merkel's conservatives in her ruling grand coalition, said strenuous efforts by countries like France and Italy to reduce their fiscal deficits came with political risks.

"I once asked the chancellor, what would be more costly for Germany: for France to be allowed to have half a percentage point more deficit, or for Marine Le Pen to become president?" he said, referring to the leader of the far-right National Front.

"Until today, she still owes me an answer," added Gabriel, whose SPD favors a greater focus on investment while Merkel's conservatives put more emphasis on fiscal discipline as a foundation for economic prosperity.

The SPD is expected to choose Gabriel, their long-standing chairman who is also economy minister, to run against Merkel for chancellor in September's federal election, senior party sources said on Thursday.