According to Viva Rio on Friday, Mandela's lawyer sent a letter to Brazil's pro-gun group Front for Legitimate Defence, asking it to stop using Mandela's image in its ads urging people to oppose the ban in a nationwide referendum on Sunday.
About 122 million Brazilians are expected to vote in Sunday's referendum, which asks voters whether or not they support a nationwide ban on firearms and ammunition, with limited exceptions.
Representative Alberto Fraga, the president of the Front for Legitimate Defence, denied receiving any communication from Mandela's lawyer, Dan MacRobert.
A telephone message was left with MacRobert in South Africa seeking comment. Viva Rio on Friday distributed a copy of the document it said it had received from MacRobert, demanding that the Front for Legitimate Defence stop using Mandela's picture in the campaign.
In the letter, MacRobert said Mandela was furious to learn that his image was being used this way and threatened to use all legal means to stop it.
"It is incorrect, improper and illegal to use Mr Mandela"
Lawyer for Nelson Mandela
"It is incorrect, improper and illegal to use Mr Mandela as I have described above - the image or picture; and then the reference to Mr Mandela fighting against apartheid: when such struggle bears no relation to the issues described - the sale of guns," MacRobert wrote.
The Front for Legitimate Defence has been using images of Mandela, China's Tiananmen Square and the Berlin Wall to convince voters that guns are essential for freedom.