Almost 3,500 convenience stores boycotted Vodafone yesterday by refusing to sell its top-ups after the mobile phone operator announced it was cutting retailers' commission by one percentage point.

Led by Spar retailers, they threatened further action if Vodafone slashed commission on sales to 5.5% from the end of this month as planned. The four major UK mobile operators, whose commission rates are similar, have twice cut rates by 0.5 percentage points in the past three years.

If Vodafone cut its rate, they all would, warned boycotters. "In the past, when one mobile phone operator reduced its commission the others have followed suit," said Chris Lewis, Spar UK's trading director for licensed products. "If that happens, Spar retailers would lose about £1.5m a year."

Spar managing director Jerry Marwood met with Vodafone on Wednesday in a bid to resolve the issue without success. Friday's Spar-led boycott was also supported by stores with Costcutter, Mace, Nisa-Today's and Martin McColl.

"I find it arrogant beyond belief," said protest organiser Kevin Hunt, who owns 28 Spar stores. "The top-up percentage is already at an absolute minimum. We can't absorb that sort of cost. The others will follow within weeks. If they do, I'll lose about £25,000 a year."

Vodafone, whose profits rose 14% to £35bn for the year to the end of March, has 18 million customers in the UK, 60% of whom are 'pay as you talk'.

"It's been a very difficult decision and not one we have taken quickly or lightly," said a spokeswoman. "We are doing more than ever before to improve the range of ways people can use Vodafone, therefore stimulating more top-ups."

PayPoint and Payzone, which broker deals between mobile operators and retailers and take a share of the commission, have been criticised for not absorbing a share of the rate cut. They said they had approached Vodafone on behalf of retailers.