Liza Bruce - known as the "Queen of Cling" - said M&S had copied her exclusive range and was selling for pounds 21 swimsuits she sold for pounds 120 in up-market shops.
Miss Bruce said she complained to M&S executives who denied copying her styles and told her they could find "no substance" to her claims.
Now she is taking legal action to make M&S withdraw the range, which she fears will cost her customers because they do not want to look like everyone else on the beach.
Miss Bruce - who counts among her customers pop superstar Diana Ross and model Christie Brinkley - is known particularly for designing figure- hugging swimwear made from a lycra-crepe material produced exclusively for her.
She said she was alerted by a customer who saw a costume identical to her blue and green dip-dyed suit at M&S in Marble Arch, central London. "I couldn't believe it when I saw what looked to me exactly like my swimsuit hanging there," Miss Bruce said.
"That blue swimsuit was a design classic which would have sold for years. Now it has been cheapened in the eyes of myself and my customers."
Her solicitor Mark Stephens said: "The suits are identical in colour, material and design. The only difference is the quality and the fact that the M&S ones have been adapted for larger-hipped women.
"This type of thing could put a designer out of business."
A spokeswoman for M&S said: "There has been correspondence between solicitors and M&S alleging breach of copyright.
"We take these allegation very seriously but after a full investigation, we have found absolutely no substance to these claims. As far as we are aware, no one has a monopoly on this fabric or on the dip-dye technique. Our suppliers have been using the same fabric for a number of years."Reuse content