The News of the World‘s former chief reporter Neville Thurlbeck claims he has refused a police offer to give evidence against his former employers.
Thurlbeck said detectives asked him to become a prosecution witness after seizing documents from his home.
Evidence taken from his property after his arrest in April indicates he warned senior colleagues two years ago about hacking, he claims.
The 50-year-old said detectives hoped he would become a key witness in return for potential immunity from prosecution.
But Thurlbeck said he turned down the offer from police last Friday, claiming he will be exonerated.
He said: “I have informed Scotland Yard that while I fully understand and respect the reason for their request of me to give evidence for the Crown in any prosecution arising from Operation Weeting, it is my opinion that a detailed and forensic inquiry into my working methods by what is a highly professional police unit will fully exonerate me.
“So, on that basis, I have declined their offer.”
A Scotland Yard spokesman said the force was not prepared to discuss Thurlbeck’s claim.
Thurlbeck, who was bailed on suspicion of unlawfully intercepting mobile phone voicemail messages, has also lodged a claim for unfair dismissal against News International.
News International chairman James Murdoch is due to appear before MPs for a second time on Thursday morning.