Shapiro, 51, is furious about published Channel 4 programme notes for Brit Girls - One Hit Wonders describing her shining "brightly but briefly" in the Sixties.
The star, who had number one hits with "You Don't Know" and "Walkin' Back To Happiness" in 1961 plus a string of other chart entries and who currently has five albums on release, is supported by actors' union Equity. She fears the show, to be broadcast on 20 December, will affect sales of her new album and forthcoming 50-date UK tour.
Channel 4 changed its programme notes and retitled the documentary Girls On Top after Ms Shapiro complained about her inclusion in the series. The notes described her as "famously written off at the age of 15".
But the originals appeared in two national newspapers, on the Internet and in a listings magazine.
Her husband and manager, John Williams, said: "This is doing tremendous harm to Helen's career and we believe it is a clear defamation regardless of what is now broadcast. "She is being associated with people who had one hit and fell into obscurity. Helen has never stopped working.
"Helen had two number one hits and her gospel album has sold over 45,000 copies. She hardly died a death by 1962."
A Channel 4 spokesman said: "It is our understanding that the issue has been settled."
Girls on Top also includes Millie Small, the Jamaican singer who had a hit with "My Girl Lollipop", PP Arnold who sang the "First Cut is the Deepest" and Billy Davies, who had a hit with "Tell Him" in 1964.Reuse content