Sources from the ruling center-right coalition said 51-year-old von der Leyen, who would be Germany's first female president, was Merkel's preferred candidate to succeed Koehler, who shocked the country with his resignation on Monday.

Several leading German newspapers also said von der Leyen, a mother-of-seven who is one of Merkel's most popular ministers, was likeliest to get the nod in an election for the largely ceremonial post due to be held by June 30.

Appointing von der Leyen might enable Merkel's conservative Christian Democrats (CDU) to retain control of Germany's most populous state, North Rhine-Westphalia (NRW). The CDU won a May 9 election in NRW by a few thousand votes, but its support plunged and the party's incumbent Premier Juergen Ruettgers risks losing power in the state.

The CDU is still in talks to form a government in NRW with the main opposition Social Democrats (SPD), but the center-left SPD is understood to be reluctant to serve under Ruettgers.

Pushing von der Leyen into the president's office could enable Merkel to replace her with Ruettgers or give him another Cabinet post, according to reports. Some papers said Ruettgers could also become Merkel's chief of staff in her chancellery.

A formal nomination for the job by Merkel's conservatives — who hold a majority in the federal assembly that would elect the new president — is expected this week. The SPD may yet put forward its own candidate for president.

Other potential candidates among Merkel's conservatives are Finance Minister Wolfgang Schaeuble, 67, and Norbert Lammert, 61, speaker of the Bundestag, the lower house of Parliament.

The stakes were raised in NRW this week after the Free Democrats, Merkel's junior coalition partners at national level, said they were prepared to consider helping the SPD form a government in NRW, reversing their earlier position.

The SPD leadership said on Tuesday evening it was keeping its options open over the NRW coalition but that it wanted to decide soon on whom to work with.

Koehler, a former head of the International Monetary Fund, stood down after a wave of criticism of his comment that German military action abroad could serve the country's economic interests.

His resignation was a blow to Merkel, whose conservative bloc has seen its support drop to 30 percent, the lowest level in nearly four years, according to a new poll.