Annegret Kramp-Karrenbauer, left, is congratulated by Angela Merkel after receiving the most votes to become the next leader of the German Christian Democrats (CDU) at a federal congress of the CDU on December 7, 2018 in Hamburg, Germany.

Annegret Kramp-Karrenbauer, the leader of Germany's Christian Democratic Union (CDU) and the one-time favorite to succeed Angela Merkel, confirmed Monday that she will not run for chancellor.

Reports of Kramp-Karrenbauer's departure came earlier in the day, but she later gave a press conference confirming the news.

Known colloquially as "AKK," the CDU leader said she will remain as defense minister until the end of the legislative period and chair of the CDU until another candidate is found. A leadership contest is expected to take place in summer.

She said that the question of who will run for chancellor was "weakening" the CDU and said her departure would have no impact on the stability of the governing coalition made up of the CDU, its Bavarian sister party the Christian Social Union (CSU), and Social Democratic Party (SPD).

The announcement throws the CDU's political future, and that of Europe's largest economy, into doubt once Merkel steps down from the chancellorship in 2021.

Kramp-Karrenbauer had taken over the party leadership in December 2018 and was widely seen as a successor to Merkel, who has led Germany since 2005.

The move comes at a difficult time for the awkward and troubled CDU/CSU-SPD coalition, however, and the CDU has seen its popularity decline in several regional elections in recent months.

There was uproar last week when the CDU party in the small state of Thuringia joined forces with the far-right, anti-immigration Alternative for Germany (AfD) party to elect a new state premier. The politician was from the Free Democratic Party (FDP) but he has since stepped down from the role following the furor caused by the vote.

However, the move sent shockwaves to Berlin as mainstream parties had vowed not to work with the AfD before now. The decision by the local branch of the CDU to help a candidate into office, alongside the AfD, was seen as a defiant message against the party's leadership — and a reason for the departure of Kramp-Karrenbauer.

Christian Schulz, director of European economics at Citi, said Monday that Kramp-Karrenbauer announced her intention to step down in a CDU board meeting on Monday "and cited divisions over cooperation with the far-left and far-right parties after the shock event in Thuringia" with Merkel reportedly favoring cooperation with the Left Party in Thuringia, which Kramp-Karrenbauer ruled out.