Nationwide building society this week hit out at the creeping plague of cash machines that penalise unsuspecting members of the public by charging up to £5 a time.

Nationwide building society this week hit out at the creeping plague of cash machines that penalise unsuspecting members of the public by charging up to £5 a time.

Four years ago the main high-street banks backed off charging for use of automated teller machines (ATMs), but since then more than one in three machines charge and the number has grown from 13,000 to 18,500 in the past six months.

Britain's biggest bank is behind the most aggressive expansion. Royal Bank of Scotland has bought Hanco, the largest operator of fee-charging ATMs. And HBOS and Abbey National have sold non-branch machines to fee-chargers.

Stuart Bernau, a Nationwide executive director, said: "This hits the poor hardest, because it is usually a flat rate. This is a serious issue for consumers, particularly as machines that were free are being replaced with ones that charge, so it is easy for people to be caught out and face an unexpected fee."

Mr Bernau said Nationwide believes that free cash machines represent best practice, whether at bank or building society branches or in places such as stations, shopping malls or pubs. However, he conceded that charging machines were justified where free machines would not be worthwhile.

Martin Salter, Labour MP for Reading West, is taking up the issue. He said: "The number of machines that charge is increasing at an alarming rate. I'm particularly concerned about the impact that charging machines have on the people least able to afford the fees. Over the longer term, the big worry is that more may follow the lead of HBOS and sell on some of their free machines to charging networks."

Nationwide wants the industry to agree to a Cash Machine Code of Practice that would impose a £1.50 cap on charges, "clear and prominent" warnings before a card is inserted, and all non-financial transactions such as account queries remaining free.