The row over fee-charging cash machines was reignited yesterday when Britain's biggest independent ATM operator revealed it is enjoying bumper profits from 275 formerly free-to-use machines it bought from HBOS.

This is partly because some of the machines charge as much as £1.75 per withdrawal, and yet people are coming back again and again. Today, Nationwide building society - a long-time campaigner on the issue - is expected to fuel the debate by announcing proposals to make it easier for consumers to tell which cash machines impose charges.

There have already been warnings from MPs and others that Britain's network of free-to-use cash machines could be in danger of disappearing altogether if the explosion in the number of ATMs that charge fees carries on unchecked.

Five years ago, virtually all cash machines in Britain were free to use but recent figures show that about 20,000 of the 54,000 ATMs - almost 40% of the network - charge a typical fee of £1.50 per withdrawal.

Campaigners claim the problem has been exacerbated by high-street banks selling off some of their ATMs to independent operators. HBOS sold 816 of its machines to Cardpoint last year, and in a trading update yesterday, the company announced that 275 of these have so far been converted to fee-charging ATMs.

Mark Mills, Cardpoint's chief executive, said: "The profit on each of these 275 machines, once converted to charging, has been higher than anticipated due to both higher retention and charging rates with excellent future visibility." Retention refers to those returning to use a fee-charging machine.

As a result, the total cost of the acquisition has ended up at around £52m rather than the £77m envisaged at the time.

Mr Mills said that in addition to the 275 charging machines, it was currently running about 300 of the former HBOS machines on a free-to-use basis, but added that these "may provide an opportunity to convert to charging at a later date". It is understood that £1.50 had been envisaged as the fee for the former HBOS machines, but that some with high "footfall" levels were managing to pay their way even with a £1.75 charge.