Source BBC News:
"We are not representing her any more," said a spokeswoman for law firm Mishcon de Reya. No reason was given.
The news follows a string of TV interviews given by Mills in which she has criticised the tabloid press.
In her latest appearance on GMTV on Thursday, Mills said she had found it "cathartic" to be able to put her side of the story to the public.
She said her estranged husband Sir Paul had not "said a word" about her comments.
Mills also vowed to continue her campaign to create "fair journalism".
Speaking out, she said, had been "the difference between being in prison and being on parole... I got the chance to defend myself".
"I've had 18 months of the worst press ever and I got the chance to protect myself and my daughter," she added.
The singer has also criticised Sir Paul in some of her interviews, calling him a "skinflint" and blaming him for their split.
Mills' divorce from the ex-Beatle is going through the courts.
Sir Paul's spokesman was not immediately available for comment.