A number of republicans, among them the Sinn Féin chairman, Mr Mitchel McLaughlin, have been warned by police that their details are in the hands of loyalists.
A Sinn Féin spokesman confirmed that Mr McLaughlin had been contacted by PSNI detectives in Derry on Saturday who advised him on personal security. Other republicans were also told to review security measures around their homes.
According to the spokesman, they were later informed of an arrest in connection with the discovery: "A number of people were contacted last night and told that a senior loyalist had been arrested with their details in his possession."
A 38-year-old man is due to appear at Derry Magistrates Court today charged with possession of information likely to be of use to terrorists. According to security sources, the man is a leading member of the Ulster Political Research Group (UPRG), which is linked to the UDA.
It is understood the man was stopped by police on Friday and found to have personal details of a republican on him. During follow-up searches of his home and office, police discovered the names and addresses of a number of other republicans.
A spokesman for the UPRG, Mr John White, said he was surprised at the arrest of the man. "He has certainly made a very positive contribution."
An SDLP councillor and former mayor of Derry, Mr Pat Ramsey, said the find was a "worrying development".