The FPB also wrote to Focus DIY chief executive Steve Johnson demanding an explanation.
One firm said it was laughable that one of Focus's letters blamed the need to increase its share on distribution costs, increases to the minimum wage, utility bills, insurance contributions and fuel duty.
He said: 'The negotiations are not between equals. I am faced with having my contract rewritten mid-contract and if I don't like it then I lose the contract altogether.
'Small firms are being hit by rising costs too and find them far harder to absorb than Focus. Now we are being hit with a double whammy because the big boys are trying to pass their increased costs on to us.'