The criticism comes since CDU, in a notable manner, on Wednesday took help from the far-right AFD to get a motion on migration policy through.
Merkel accuses her successor as CDU leader, Friedrich Merz, of having abandoned a promise from last autumn not to take help from the far-right.
"I see it as wrong not to feel bound by this proposal and thereby, on January 29, 2025, for the first time, consciously allowing a majority with AFD's votes in the federal parliament", Merkel writes in a statement on her official website.
Merkel, who resigned after the 2021 election, instead urges "all democratic parties" to work together "honestly and factually" to prevent violent acts.
The motion in the federal parliament calls for tougher border controls and deportation of persons who do not have the right to stay in Germany.
