Blake Shelton Threatens Magazine Over Affair Allegation ... I Never Touched That Woman

Blake Shelton is slamming rumors he had an affair with country singer Cady Groves ... threatening to sue the magazine that made the claim ... and also insisting she's in on the lie.

We've learned Shelton's attorneys have fired a letter to In Touch magazine, demanding they retract their story about Shelton and Groves carrying on an affair.

In the letter, we're told Shelton's attorneys describe Groves as someone who became "infatuated" with Shelton. The letter accuses the magazine of using Groves as its source, and any insinuation by her that she had an affair with Blake is a lie.

The legal letter is strong, "Mr. Shelton did not have a sexual relationship or affair of any nature whatsoever with Ms. Groves."

Since the rumor surfaced, Groves has spoken about Shelton ... but has stopped short of denying the affair.

In Touch is not backing down, saying, "Not only does In Touch magazine stand by its story but we have another exclusive about Blake and Miranda's divorce in this week's issue."