"Many thanks for your letter... I am very happy that you stand vindicated in the 2G case. You and your family have suffered greatly in this process but all your friends are greatly relieved that truth has prevailed," he said in a reply, dated January 2, to Raja's December 26 letter.
In his letter, Raja, of the DMK, had said that he "understood the compulsions" that prevented the then Prime Minister from "openly supporting him".
A special court on December 21 acquitted Raja and Kanimozhi of the DMK as well as businessmen and executives accused of bribery in the 2008 allocation of 2G telecom spectrum, a "scam" that allegedly contributed to the Congress-led UPA's defeat in the 2014 parliamentary elections.
"I also understand the compulsions that prevented you from openly supporting me. Today, I stand vindicated.
"I hope you will acknowledge that I remained loyal and faithful to you - unlike some senior cabinet colleagues - and ensured that you did not suffer personal embarrassment in the trial proceedings."
"Now that the truth about 2G is out in the open, perhaps you too could come forward in my support, which you could not earlier," Raja said.
He noted that it was on this very day (December 26) a decade ago, he had written to the Prime Minister on the subject of issuance of new UAS licenses and allocation of 2G spectrum.
"Neither of us could have imagined the twists and turns in our political and personal lives over the next ten years. A few powerful individuals launched a propaganda machine, magnified a thousand times by a complicit and pliable media, and were able to mislead even Parliament and the apex judiciary.
"It cost you the UPA government, and it took seven years of my life, including fifteen months in jail," he wrote.