Angela Merkel has announced she will not seek another term as German chancellor when her mandate finishes in 2021, ending more than a decade in which she has dominated European politics.

Speaking after disastrous regional elections in Hesse and Bavaria for her Christian Democrats and its Bavaria-only sister party, Merkel said she saw the results as a "clear signal that things can't go on as they are".

She said would not stand as party leader at the CDU conference in December and at the next elections in 2021 she would not seek another term as chancellor, announcing her complete withdrawal from politics after that date.

Merkel said national politics had had a regrettable negative influence on the results in Hesse, calling them "disappointing and bitter". The CDU slumped to 27% in preliminary results in the state, the party's worst showing in the state since 1966 and a drop of 11 points since Hesse last went to the polls in 2013.

The announcement she will not seek re-election as CDU chair in December kickstarts the race to replace her as CDU candidate for chancellor in 2021, when the next federal election is due.

Merkel said her 13 years as chancellor had been a daily challenge and an honour, but she recognised it was time to start a new chapter.

It had been widely assumed that this would be Merkel's final term as chancellor but she had not confirmed that herself. She said on Monday she has made the decision before the summer recess and had planned to announce it next week.