Federal police are refusing to release an uncensored version of the National Broadband Network letter that sparked its controversial media leak investigation, claiming it could endanger lives and threaten national security.

A heavily redacted version of the NBN Co document has been provided to Fairfax Media under Freedom of Information laws.

The letter from the NBN to the Australian Federal Police is dated December 9 - two weeks after Fairfax Media published the first in a series of reports based on leaked documents about cost blowouts and delays.