The senior German conservative politician Norbert Röttgen has put himself forward as a surprise candidate to take over as leader of Angela Merkel’s Christian Democrats (CDU), further complicating the party’s chaotic succession planning.

Last week, Annegret Kramp-Karrenbauer, Merkel’s protege, threw the party into turmoil by saying she would not stand as chancellor in the next federal election, due by October 2021, and announcing she would give up the CDU chair.

Röttgen’s unexpected move makes him the first official, and fourth possible, contender for the post of party leader. He has something of a national profile due to his role as head of the parliamentary foreign affairs committee but is not seen as a big hitter in the party. He also served as an environment minister under Merkel from 2009 to 2012.

In the past, Röttgen has been seen as someone who could work with the Greens, currently the second-biggest party in the polls after the conservative bloc.

In a letter quoted by the regional newspaper Rheinische Post to Kramp-Karrenbauer, Röttgen said the situation was “so serious that it was about the future of the CDU and what that meant for the stability of Germany”.

The other potential CDU leadership contenders are Friedrich Merz, an economic liberal and former arch-rival to Merkel, the rightwing health minister, Jens Spahn, and Armin Laschet, the premier of North Rhine-Westphalia, Germany’s most populous state. A centrist, Laschet is seen as a continuity contender.

The CDU leader is likely to be chancellor candidate for the conservative bloc, but the Christian Social Union – the Bavarian sister party to the CDU – could put forward a candidate.