The files from Operation Motorman, which was conducted by the Information Commissioner's Office in 2003, have been requested by the police conducting the current investigation.
The scale of the dubious news gathering practices is evident from the earlier inquiry, which revealed that 4,000 requests from 300 journalists and 31 publications for confidential information were made to a private investigator.
Many of the cases indicated use of illegal practices, according to a BBC report. The investigation found the Daily Mail had made the most requests, followed by the Sunday People and the Daily Mirror.
The Daily Mail said the information obtained may have been for reasons of public interest, and Trinity Mirror Group said its journalists worked within the law and the Press Complaints Commission code of conduct.
The current operation, codenamed Operation Weeting, has been boosted by increasing the number of police officers working on it from 45 to 60 officers, Scotland Yard said.
Metropolitan Police Deputy Assistant Commissioner Sue Akers said the move came after a "significant increase in the workload" over the past fortnight.
Akers guaranteed a thorough inquiry into the issue during a hearing of the Home Affairs Select Committee last week.
Akers said there had been a "surge of enquiries and requests for assistance from the public and solicitors".
She said: "I have said all along that I would keep the resources under review and this has led to the increase.
Similarly, if the demand decreases, I will release officers back to other duties".
On the increase in strength working on the inquiry, Home Affairs Select Committee chairman Keith Vaz said: "This is excellent news. The extra resources will assist to help move things along much more quickly".