Regulators have vowed to take action after it emerged an elderly man was conned out of thousands of pounds as a result of charities buying and selling his personal details.
Widower Samuel Rae's details were passed on hundreds of times after he forgot to tick a box on a lifestyle survey 21 years ago.
The 87-year-old former army colonel's data has since been given to charities, and even some companies associated with scams with the result that he has lost almost £35,000, an investigation by the Daily Mail found.
Mr Rae's son Chris said his father, who lives in St Buryan in Cornwall and suffers from dementia, had been treated in an "absolutely disgraceful" way by charities.
It comes just months after the Government commissioned a review into the way charities carry out their fundraising following the suicide of Britain's longest-serving poppy seller Olive Cooke.
Mrs Cooke, 92, from Fishponds in Bristol, had told how she was receiving up to 267 letters in a month and regular phone calls from charities asking her for donations.
'Plagued with unwanted mail'
Information Commissioner Christopher Graham told BBC Radio 4's Today programme his office would be investigating and said: "If the law has been broken, we will take action.
"Whether or not Samuel Rae ticked the box in 1994 and is still being plagued with unwanted mail and unwanted approaches is beside the point. The Data Protection Act is very clear - the very first principle is that your data is only processed fairly and lawfully. What's described in the papers this morning doesn't look like that."
A failure to tick a box "isn't consent and it doesn't give you the right to trade in people's personal information years after the event", he said.
Mr Graham said: "If there's any connection between the good work that charities do and the scam merchants, that's very concerning and we've got to get to the bottom of how this information was passed on".
But he warned: "There's a danger here of blackening a whole sector. Charities seem to be becoming the new dirty word, and that clearly isn't fair. We've got to look at the specifics of the case and work out exactly what has happened and if the law has been broken, we will act."
The Information Commissioner's Office's (ICO) head of enforcement, Steve Eckersley, added: "We have been presented with some clearly concerning findings about data sharing and sale in the charity sector, and we will be investigating them further.
"The law applies to charities as it does to any other company. We already have an ongoing investigation into marketing practices in the charity sector, and where we find companies have broken the law we will take action."