The Nationwide is threatening to sue Barclays Bank over charges for using its cash machines according to a report by the BBC.
The building society says it will take the action unless Barclays drop the charges for Nationwide customers using its ATMs.
Barclays has angered rivals by introducing the charges of £1 each time their customers use a Barclay's machine.
Nationwide claims the charges which take effect on September 23, contravenes a reciprocal agreement for the use of the Link network of bank and building society cash machines.
Nationwide claims the move is against customer interest, but Barclays says the decision was taken so it could focus on its own customers.
It has dropped charges its customers making withdrawals from non-Barclays cash machines.
Most other banks still charge their customers around £1 to £1.50 for using many other banks' machines.