NI says the move restores retailers' margin to its standard 23.2%, but in a letter sent to Sun readers via newsagents, Mail Newspapers accused it of "reducing the profit your newsagent earns by 15% to simply boost [its] own profits".
"This is a hammer blow to the hard-working newsagents of Britain," it added.
However, the message rather undermined its support for local stores by offering new Mail subscribers a £50 supermarket voucher.
Retailers have challenged NI's move in the belief that other publishers will follow its lead in cutting margins unless newsagents stand up for themselves an outcome that could see a devastating £120m a year clawed back from stores.
Stefan Wojciechowski of the National Federation of Retail Newsagents, which is prevented by law from organising collective retaliatory action, said: "There is nothing that we can do, or would want to do, to stop individual retailers from reacting to protect their businesses."
Newsagents contacted by Convenience Store said they were refusing to accept vouchers for NI titles, placing The Sun beneath other papers and promoting rival news titles. A number had increased the delivery charge for The Sun.
John Lennon of the Association of News Retailing warned: "There may well become a point when it is no longer economically viable to sell a title if the cash received is significantly less than rivals."
East Anglian newsagent Brian Webb is calling on retailers to sign a petition at www.webbsofleverington.co.uk.
"We intend to show the news industry that we will make the strongest representation against any publisher whose action seriously affects our news business," he said.