Daphne Barak, the journalist claiming Kevin Federline linked Britney Spears to crystal meth, has come under heavy fire from the former couple ... but she's not backing down, insisting K-Fed made the statement, despite his denial.
A spokesperson for Mail on Sunday tells us Daphne and her team spent months working on a documentary with Kevin, his family and Britney's two sons -- Sean Preston and Jayden.
The rep says, "Daphne Barak has assured us that the quotes attributed to Kevin and the family are accurate and fairly reflect the interviews conducted, in which Kevin expressed his fears that Britney might be using crystal meth to Daphne and to her producer Erbil Gunasti and members of the film crew. He did so clearly and repeatedly."
They continue, "Kevin also made it clear to Daphne that he hoped someone would expose the situation, as he saw it, because that would be the way to get help for Britney."
What's more -- they say they reached out to Britney's reps for comments on the meth allegation ... but got no response.
For his part, Kevin's attorney, Mark Vincent Kaplan tells us in response, "Mail on Sunday knows no more than what they were told by Daphne. Accurate reporting might mean they accurately took down what Daphne told them but in no way means what Daphne told them was accurate."
TMZ broke the story -- Federline went scorched Earth on The Sun and Daily Mail Sunday after a report claimed he stated about Britney, "I fear she's on meth – I've been praying someone would make it public and that she wakes up ... It's terrifying. She is the mother of my boys."
After the story came out, Kevin told us, "It saddens our family that Daphne Barak and Erbil Gunasti have decided to fabricate lies and publish the heart ache our family has endured, along with the trauma of our minor children."
Federline, however, never specified which part of the story was fabricated.