SÃ O PAULO—Brazil's BTG Pactual was reeling anew on Monday, after a top federal prosecutor said Brazilian investigators allege they have evidence the bank paid a powerful lawmaker more than $11.5 million in exchange for political favors.

Brazilian investigators say they have obtained a document showing that BTG in 2013 paid a bribe of 45 million reais to House Speaker Eduardo Cunha for including an amendment in a piece of legislation, which would boost the bank's profits. Mr. Cunha is already under investigation by authorities for allegedly stashing millions in suspected bribes in Swiss bank accounts, according to a court order seen by The Wall Street Journal.

Mr. Cunha denied the allegations. "It´ s a true absurdity," Mr. Cunha said in a statement Monday.

The new allegations come hours after BTG Chairman and Chief Executive André Esteves resigned his post following his arrest last week in a widening of the corruption investigation involving Petró leo Brasileiro SA. Authorities jailed Mr. Esteves on allegations of witness tampering and obstruction of justice. Prosecutors allege Mr. Esteves tried to buy the silence of a key witness whose testimony could potentially implicate the banker in the scandal.

Mr. Esteves' lawyer said last week that Mr. Esteves denied the allegations and planned to fight the charges. Mr. Esteves' lawyer declined to comment Monday because he said he hasn't yet had access to the new court document.

BTG said in a statement Monday that it is cooperating with the authorities and that the bank "vehemently denies having made any type of payment for an alleged benefit" related to a banking law that was approved by Congress in 2013.

Monday's news is another blow to BTG, which last week faced a wave of withdrawals by clients worried about fallout from Mr. Esteves' arrest.

Longtime BTG partner Persio Arida last week stepped in to lead the bank. On Sunday he was named Executive Chairman, while partners Marcelo Kalim and Roberto Sallouti were appointed co-Chief Executive Officers. Changing the executive structure of the bank was the first step in an effort to regain market confidence, according to a person close to the bank.

Other steps may include the sale of assets, a second person with knowledge of the situation said. BTG is in talks to sell a 12% stake in hospital chain Rede D´ Or Sã o Luiz SA to Singapore's sovereign-wealth fund GIC, the person said.

The scandal will possibly accelerate the sale of Rede D´ Or and other assets, according to banking analyst Luis Santacreu, at Austin Rating in Sã o Paulo.

Requests for comments to GIC weren't answered.

There is also speculation that current partners may try to buy out Mr. Esteves' nearly 30% stake in BTG to distance themselves from the embattled former executive, said two other people close to the bank. One of the people, however, said no decision on possibly buying out Mr. Esteves has yet been made.

Unit shares of BTG were down 8.53% in the morning in Sã o Paulo, trading at 20.90 Brazilian reais. The price of BTG's traded units fell more than 30% since the close of trading on the day before Mr. Esteves's arrest, slumping from 30.89 reais to 20.23 reais at around midday on Monday.

Write to Luciana Magalhaes at Luciana.Magalhaes@dowjones.com and Rogerio Jelmayer at rogerio.jelmayer@wsj.com

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(END) Dow Jones Newswires

November 30, 2015 11:45 ET (16:45 GMT)