EveryChild has updated its procedures and begun retraining its 15 street fundraisers after a folder containing details of donors' direct debits went missing on a Norwich street.
Mike Smith, who has lobbied for tighter controls on street fundraising, found the folder and sent copies of its contents to the Information Commissioner's Office, which is conducting an inquiry.
A statement from EveryChild, which seeks sponsorship for children in developing countries, said it had completed its own investigation and apologised to the five people whose data was lost.
"It is likely that a folder containing the forms went missing from the fundraisers' belongings, which were placed nearby," the statement said. "While it is common practice for street fundraisers to keep their fundraising materials on the street when they are speaking to members of the public, they are asked to ensure that their folders and forms should never be left unattended at any point.
"EveryChild regrets the mistake that occurred on this occasion and is strengthening procedures to prevent any future breach of data security." The charity said it was confident such an incident would not happen again.