Annagret/Angela
© Matthias Rietschel/ReutersAnnagret Kramp-Karrenbauer • German Chancellor Angela Merkel
Angela Merkel's designated successor has announced she is not planning to run for the German chancellorship at the next federal election and plans to step down as leader of the conservative Christian Democratic Union (CDU), German media reported on Monday morning.

The surprise announcement comes in the middle of a major row over the centre-right party's "firewall" against the far-right, after CDU delegates in eastern Germany defied the party headquarter's ban on cooperating with the Alternative für Deutschland (AfD).

Annegret Kramp-Karrenbauer, 57, won the contest to succeed Merkel as leader of the CDU in December 2018 and was seen as the candidate most likely to continue the current German chancellor's centrist course. But "AKK", as she has come to be known in German media, has struggled to build a profile in Merkel's shadow, even after doubling up as defence minister last July.

Questions over her control over an increasingly divided CDU returned to the fore last week, when politicians from the party's branch in Thuringia voted with the AfD to oust the state's premier Bodo Ramelow, from the leftwing Die Linke party.

Merkel's unusual decision to intervene in the affair, announcing that it was "unforgivable" for democratic parties to win majorities with the help of the AfD, accentuated Kramp-Karrenbauer's lack of authority.

Kramp-Karrenbauer's standing was further diminished when the CDU's Thuringia branch defied her call to dissolve the state parliament and call a fresh election.

On Monday morning news magazine Der Spiegel reported that Kramp-Karrenbauer had cited the unclear positions of certain elements in her party towards the AfD and Die Linke as the reason for her resignation, and reiterated that she was against cooperations with both parties.

Merkel is reported to have accepted the resignation of her designated successor, who was previously a state premier in the Saarland on the border with France, while also letting it be known that she would like AKK to stay on as defence minister.

The vacant seat at the top of Germany's most powerful party since 1945 opens up a debate over the country's political direction of travel in the post-Merkel period.

At least four frontrunners present themselves. Armin Laschet, the 58-year-old state premier of North-Rhine Westphalia and a staunch defender of Merkel's refugee policy, would be the candidate most likely to continue the outgoing chancellor's steering of the CDU in the centre of the political stream.

By contrast, the anointment of veteran rightwinger Friedrich Merz, who lost out to Kramp-Karrenbauer in the race for the party leadership in 2018, would symbolise a departure from the Merkel era and a return to the CDU's more traditionally conservative roots. Merz, who isn't currently a member of the Bundestag but quit his role at asset manager Blackrock last week, has in recent months openly criticised Merkel's "lack of leadership".

Two other candidates are less easily categorised. Markus Söder, the state premier of Bavaria, hails from the CDU's traditional more conservative sister-party, the CSU. But in recent years Söder has proved a canny and popular political operator who has emphasised his party's environmental credentials and could prepare the party for a coalition with the Green party at a political level. During last week's political earthquake in Thuringia, Söder was quickest to come out in condemnation of any form of collaboration with the far-right.

Thirty-nine year old Jens Spahn used to have a profile as a Merkel critic but has mellowed since taking over the health ministry, and recently pitched his tent closer to the chancellor's pragmatist tent, championing both "migration and patriotism" in an op-ed for the Guardian. Spahn came third in the 2018 leadership contest.