Germany’s chancellor decried “unfriendly attacks” on banks after UniCredit moved to raise its stake in the lender.

Italian government officials are growing frustrated at Germany’s opposition to a potential takeover of Commerzbank AG by Milan-based UniCredit Spa, threatening to disrupt the delicate diplomatic relations between Berlin and Rome.

The UK-based lender arranged derivatives linked to Commerzbank for the Italian lender in the weeks before Berlin sold a stake earlier this month, people familiar with the matter said. Mr Orcel subsequently also relied on firms including Barclays to ...