It emerged last week that Rhondda Cynon Taff were one of three Councils in Wales who had mistakenly sold electoral registration data to companies including credit reference agencies which included personal information that they should not have released.
The Information Commissioner will be looking into the breach and the Council have written to all residents whose details were incorrectly handled.
Welsh Liberal Democrat Councillor Mike Powell commented
"I am pleased that the Council held its hands up as soon as the mistake was discovered and reported the matter to the Information Commissioner.
"However there are questions to be asked about how this happened in the first place and I hope the Cabinet Member responsible will come to the next Council meeting with a statement on how it happened in the first place and what steps are being taken to ensure it does not occur again.
"The public needs to have confidence in Council processes and data protection is a very serious issue. Nobody wants to see a witch hunt but those responsible need to be taken to task and procedures reviewed.
"I am led to believe that the sale of electoral registers is handled by the IT department and not the elections unit who are responsible for putting them together. "
The law allows for an 'edited' version of the register to be legally sold on to third parties who use them for marketing purposes. Residents are able to opt out of this version if they so wish.