Europe's central bank just flashed a warning for any potential Deutsche Bank-Commerzbank merger

Getty Images / Mark Kolbe

The potential merger just got a warning sign from the ECB.

• The European Central Bank is adding its criticism to the potential tie-up of Deutsche Bank and Commerzbank
• Chair of the ECB's Single Supervisory Mechanism told the Financial Times: "I do not particularly like the idea of national champions."
• Berlin "has become more aggressive in protecting its largest businesses from foreign pressure," the FT said.

When Deutsche Bank and Commerzbank confirmed they are holding formal talks about a merger, critics were quick to question a tie-up that may lead to tens of thousands of job losses and the potential failure of a plan that may do little to address underlying problems at both banks.

Now, the European Central Bank is adding its criticism to the mix.

Speaking to the Financial Times, Andrea Enria, chair of the ECB's Single Supervisory Mechanism (SSM), said: "I do not particularly like the idea of national champions, of European champions; especially when you are a supervisor, you should not promote any particular structural outcome."

Enria, spoke before the two banks confirmed merger talks, the FT said on Tuesday, and didn't say how the central bank would treat the deal.

The FT wrote that Berlin "has become more aggressive in protecting its largest businesses from foreign pressure," and that Enria "made clear that in all instances, the SSM would ignore any political motivations behind proposed tie-ups."

Deutsche Bank and Commerzbank shares are both down around 2% as of 9.20 a.m in London (5.20 a.m ET).

Read more: IT'S OFFICIAL: Deutsche Bank and Commerzbank are holding merger talks - but critics worry about job cuts and patchy past deals

"The only things we care about are the sustainability of the project," he told the Financial Times.

A merger of the banks would create Europe's fourth-largest lender, with north of €1.8 trillion ($2 trillion) in assets and a market value of about €25 billion, according to Bloomberg.

Analysts think the combined group could slash its annual costs by more than €2.2 billion and command one-fifth of Germany's high-street banking business. Investors cheered the news on Monday, sending the shares of both banks higher.

Read more: Deutsche Bank's malaise goes deeper than its tumbling share price

However, the deal could result in the elimination of up to 20,000 jobs - more than 13% of the two companies' combined workforces - says the chief of Germany's Verdi labour union, according to Reuters. The union says there's overlap between the firms' retail and business customer segments, arguing international deals would suit both firms better.

Both Deutsche Bank and Commerzbank have patchy deal histories. Deutsche Bank struggled to integrate Postbank, which it bought for €6.5 billion euros in 2010.

Commerzbank's troubled takeover of Dresdner Bank in 2008 meant the financial crisis hit it harder, leading to two government bailouts. Yet mergers are to be expected in a nation with close to 1,600 banks, according to The Economist.

The mega-deal also comes at a difficult time. Both banks are navigating an economic slowdown in Germany - which nearly fell into recession at the end of last year - as well as the broader eurozone. A depressed backdrop could make it harder for the firms to satisfy Germany's tough unions and comply with its strict labor laws.

Deutsche Bank's sales fell and borrowing costs rose in the final quarter of 2018, and German officials raided its headquarters. It's unclear how absorbing Commerzbank will help Deutsche Bank's ailing investment banking arm, its main earnings driver.