Retailers say that the rise, which follows a 3.65% increase in January, breaches Smiths' promise to hold its charges for 12 months. Other wholesalers operating regional monopolies are expected to follow suit.
NFRN national president Naresh Purohit said: "Why is it that monopoly wholesalers automatically turn to their captive retail customers when they are short of cash, rather than go to the publishers who not only control cover prices but margins across the industry, too?"
He added that publishers could inject extra revenue into the supply chain by increasing the cover price of their titles.
Smiths News' chief executive Mark Cashmore said exceptional increases in costs had forced the rise. "Over time, vehicle running costs have become a greater percentage of distribution costs," he said. "In taking this decision now we have limited the increase to what we believe is a sustainable level for the next 12 months."
However, Brian Webb, who operates a news delivery service in East Anglia, said the wholesalers had to stand by the agreement set in January. He said: "They anticipate that there will be no further increases for 12 months, but how can we believe that?"
Association of News Retailers managing director John Lennon called the additional charges "completely unacceptable". He said: "They are further evidence of the abuses by wholesalers that could only happen in a non-competitive market," he said. "I can only assume that this has been done with publisher blessing - if retailers pay obscene amounts of carriage, publishers don't have to."