Germany said the offer undervalued Commerzbank, while Frankfurt prosecutors opened a preliminary market-manipulation probe.
On Tuesday, Germany officially rejected UniCredit's offer to buy Commerzbank, citing an inadequate price and concerns about the Italian bank's aggressive approach.
The Milan-based bank launched its unsolicited 35 billion euros bid in early May to gain control of Commerzbank, while the German government, holding a 12% stake, has long opposed the tie-up.
Frankfurt prosecutors opened a preliminary investigation into "possible market manipulation" on Monday, following a criminal complaint filed by Commerzbank's works council on Friday.
Commerzbank CEO Bettina Orlopp unveiled a strategic plan to boost profitability and defend independence, while UniCredit aims to merge the lender with its German subsidiary HypoVereinsbank pending European Central Bank approval.
Germany's Financial Market Stabilisation Fund stated it "supports the strategy of independence" for Commerzbank, echoing Chancellor Friedrich Merz's earlier warning that the bid was destroying "trust" in the institution.