An increasing number of shops are surcharging purchasers of pay-as-you-go credit for energy meters, according to research by the Money Advice and Budgeting Service.

MABS, which first highlighted the practice in August, has called it "a shameful rip-off of the most financially vulnerable consumers".

The Government has been encouraging 150,000 households to top-up energy cards several times a month to reduce the rate of disconnections.

Where customers owe money to utilities, around a quarter of each top-up goes towards reducing arrears.

Electric Ireland promised these consumers will not pay more than the standard rate.

However, a MABS survey earlier this month of shops in Dublin 10 and Dublin 20 found four out of five surcharging the transactions by anything from 2-5%. That is up from three out of five last May.

This month, only half the surcharging shops displayed a notice of the fee.

A spokesman for MABS said Payzone has told it that two out of three customers topping up energy, phone and other types of cards said they made at least one other purchase in the shop concerned.

He noted that Tánaiste Joan Burton had written to the Commission for Energy Regulation about energy surcharges following publicity about the practice in August, but that it had continued to spread despite this.

The Head of MABS has said there may be a need to outlaw surcharging for basic priority debts, if the issue of top-up charges on pay-as-you-go energy meters is not adequately addressed.

Michael Culloty questioned why some retailers were charging for what he described as a "basic service" when they were already getting payment from Payzone, which provides the pay-as-you-go service for energy meters.

He urged traders, Payzone and the electric companies to come together to find a solution to the problem.

Mr Culloty described the practice as a "rip-off" and "very unfair" that those on very low incomes have to pay more for gas and electricity than the rest of us.

He said the service should be free to the consumer and that charges had only crept in the past year.

Mr Culloty said it had expected the situation to improve following publicity over a similar survey last May, however he said it had not.

He said: "The highest charge was in Co Mayo where a customer was charged €1 on a €10 top-up, which is crazy. In the greater Dublin area, it varies between 20c and 50c for a top-up.

"The other thing to remember is a lot of these consumers are already paying a portion of their top-up to arrears, they could be paying 25-30% on top."