Xtina's Lawyer Attacks Website -- She's NO Child Abuser!

Christina Aguilera is furious over an Internet report implying that she beat her son Max and gave him a black eye ... and now TMZ has learned she's threatening to sue.

Aguilera's attorney, Eric Greenspan, fired off a letter to MediaTakeOut.com ... after the site published a story under the headline, "CHILD ABUSE???? CHRISTINA AGUILERA'S SON IS PHOTO'D WITH A BLACK EYE!!!"

Greenspan writes, "Christina is a wonderful mother and was not present in the park when Max fell and hurt himself."

He adds, "To imply that she had something to do with this incident, without any investigation on your part whatsoever, is reckless and unconscionable."

After threatening to pursue legal action against the site, Greenspan concludes, "HAVE YOU NO SHAME!!!!"