Angela Merkel was once revered as the most powerful woman in the world, seen by many as a crucial defender of the post-war international order and a bulwark against a rising tide of populists.

On Monday, the German chancellor signaled that her 13 years in power is coming to an end.

She told a press conference in Berlin that she will not seek re-election as chair of her own party, the conservative Christian Democratic Union, when it holds its annual congress in December.

Furthermore, she will not seek re-election as chancellor — as the leader of Germany is known — at the next scheduled election in 2021.

Many experts experts say the German chancellor is now a "lame duck" and predict she may be gone far sooner than that.

"Angela Merkel knows quite clearly that she is living through the last months or through the last year of her chancellorship," said Werner Patzelt, a politics professor at the Technical University in Dresden.

Merkel's once-imperious position began to falter after her controversial "open door" immigration policy saw more than 1 million migrants and refugees enter Germany in 2015.

Earlier this year, her party was forced into a "grand coalition" with the left-wing Social Democratic Party after both hemorrhaged votes to the right-wing populist party Alternative for Germany.

The subject of immigration has bitterly divided the country and caused infighting in the government.

Merkel's announcement — described by the tabloid Bild as a "quake" — followed more chastening losses at the ballot box this weekend. The chancellor called the results in the state of Hesse "disappointing and bitter."

Merkel has been leader of the CDU since 2000 and German chancellor since 2005. She is currently serving her fourth term as leader of the country.

In Germany, the leader of the party and leader of the country are two different roles. Merkel has always said she believes these jobs go hand-in-hand, but admitted that she was turning away from that position. So it is unclear what might happen next.

Her weakened position mean experts have been forecasting Merkeldämmerung — or "Merkel twilight" — for months now. Her announcement means her final act as leader has officially begun, according to Josef Janning, head of the Berlin office of the European Council on Foreign Relations.

"This is an acceleration of events that I think was going on as an undercurrent for quite some time," he said.

The CDU will get a new leader at the party's congress in December. Merkel confirmed that Health Minister Jens Spahn and CDU General Secretary Annegret Kramp-Karrenbauer have announced they will vie for the job.

The party will put up a new candidate for chancellor in 2021 — or, if political instability means an election is triggered, even sooner than that.

Alexander Smith reported from London and Andy Eckardt reported from Berlin.