About 83% of Commerzbank employees who took part in a staff survey are against a merger with Deutsche Bank, according to preliminary results of the poll seen by Reuters today.
Germany's two largest banks said in March they were in talks to merge, a tie-up that unions have said could result in the loss of 30,000 jobs.
Opposition to the proposed deal is stronger at Commerzbank than Deutsche Bank, where a similar survey showed nearly 70% did not favour a tie-up.
"Distance yourselves from the merger plans," Commerzbank's works council wrote in an open email to the bank's management board with the results of the survey, taken during the fifth week of merger talks between the two banks.
Commerzbank declined to comment.
Employees' opinions count in Germany because workers make up half of companies' supervisory boards that usually are required to sign off on mergers.
The strength of feeling among staff against the merger could also help labour to win concessions from management in exchange for their support.
The survey results are preliminary because Commerzbank employees have until May 3 to give their opinions in the poll conducted by the bank's headquarters staff works council.
Of 11,000 employees sent the survey, 5,500 had responded as of this morning, with many away for the Easter holiday.
Of those who responded, 82.5% said the two banks should not merge, 13.7% that they should and 3.8% had no opinion.
The potential merger with Deutsche Bank has prompted media speculation in Germany about boardroom rows at Commerzbank over the proposal.
Last week, Commerzbank's supervisory board chairman Stefan Schmittmann dismissed reports of board dissatisfaction with CEO Martin Zielke as irresponsible and unfounded.
Zielke has already tried to win over employees.
Days after the talks were announced, Zielke promised the bank's employees a quick decision on whether to go forward with a merger, according to a memo seen by Reuters.
The Commerzbank works council has since written to the board members to protest against the merger, saying the idea lacked support among workers, customers and society.
Zielke has since met with staff to put the case for the deal.
ING approached Commerzbank about possible tie-up - report
Earlier reports said that ING Groep had approached Commerzbank about a possible tie-up.
Germany's Manager Magazin, citing unnamed sources, said ING made the approach to the German government, which owns a stake in the German lender, and to Commerzbank chief executive Martin Zielke.
But Zielke declined to engage in formal talks, the report said.
The reported interest by ING comes as Commerzbank and Deutsche Bank engage in talks to merge.
According to the magazine, ING's CEO Ralph Hamers suggested moving ING's headquarters to Frankfurt in event of a deal.
He also promised to cut fewer jobs than would be the case in a Commerzbank tie-up with Deutsche, the magazine said.
Commerzbank is a ripe candidate for a foreign takeover, which is one reason why the German government has been encouraging a tie-up with Deutsche to create a national champion, sources have told Reuters.
Italy's UniCredit has said it could explore a merger with Commerzbank if talks with Deutsche fail, two people with knowledge of the matter told Reuters earlier this month.