The Mirror says the letter has been released by Argentinian club San Lorenzo, who hope to sign the unhappy Newcastle centre-back.
Last week, Newcastle bosses claimed their 31-year-old skipper was happy to stay on Tyneside.
But the letter - apparently signed by Coloccini and his agent - appeared on the official Facebook site of his Argentine suitors, claiming last week's peace deal was only a "momentary suspension" of his desire to move.
Newcastle officials are investigating the origins of the letter, which surfaced just hours after Magpies manager Alan Pardew had claimed a compromise had been agreed to keep his captain at the club for the rest of the season and possibly beyond.
The lengthy message, to San Lorenzo fans, revealed Coloccini remains homesick and thanked the club for trying to sign him.
It is addressed "To my San Lorencista family".
The letter reveals: "I write these lines from a long way away where I am missing my country, my family and my close ones. And with sadness of knowing that after a series of meetings held here in Newcastle that my return to Argentina has been momentarily suspended.
"Unfortunately, life does not always end up like you wanted it or needed it to, many times is it influenced by third parties and the rigidities of contracts that are stronger than the will of one's own desires."