The Communications regulator ComReg has expressed "serious concerns" about Vodafone's plan to switch customers to online billing without their consent.
The mobile operator recently began informing customers tariff that they would in future be receiving their bills online rather than by post.
The move is expected to affect just under 200,000 of the company's 2.4 million customers.
However, Comreg has criticised the plan and reminded the firm's customers that they are entitled to receive a paper bill from their mobile operator.
"The decision to move to electronic billing should only be made with the prior explicit agreement of that consumer," ComReg said in a statement.
The regulator wrote to the company earlier this week to inform Vodafone that it would be violating the terms of its licence if it proceeds to automatically switch customers to online billing. It has called on Vodafone to hold off switching subscribers until a consultation process on online billing is completed.
The consultation process was launched after a similar move by rival operator O2 last October resulted in the communications regulator threatening the company with court action if it continued to switch customers to online billing.
O2, which has 1.7 million customers, said at that time that more than 70 per cent of its post-paid customers pay their bills online, with the rest having opted out by informing the firm they want to continue receiving paper bills in the post.
St Vincent de Paul and the Money Advice and Budgeting Service (MABS) have both said they have concerns about people's ability to manage their finances without receiving a paper bill.
The Consumers Association said Vodafone’s plan was "anti-customer and detrimental to those who do not have internet access or are uncomfortable using it."
"We believe Vodafone plan is being implemented in the belief that people will not bother to ring and ask for a paper bill and that ComReg will move too slowly against them," said Dermot Jewell, director of the association.
In a statement Vodafone said it was following in the footsteps of rival operators in switching subscribers to online billing. It said that the move was also influenced by customer feedback.
"Customers have told us that they prefer to receive online bills because of the convenience and reduced impact on the environment," the company said.
"While we are keen to promote the benefits of online billing, we fully recognise that it doesn't answer all customers' needs and it remains optional for Vodafone customers to receive paper bills."
The company said that mobile customers who decide to continue to receive paper bills would not be charged for them.
It added that savings achieved from switching customers to online billing this year had been put towards a country-wide tree planting programme and to support the Tree Council's National Tree Week initiative.
