Natasha Hamilton has reportedly said that she does not want the group to reform due to arguments between members.
The 29-year-old is said to have refused to rejoin the band if both Jenny Frost and Kerry Katona return, reports The Sun.
Frost recently spoke out against Katona's possible inclusion in the reunion plans last month.
An insider said: "Jenny has already said she will not rejoin if Kerry is in it. The claws really are out.
"Jenny will come back in a trio with Natasha and Liz McClarnon - but nothing else."
The girl group announced last month that they will reform for a series of gigs this summer, possibly including the Queen's Jubilee concert in June.
Katona recently said that it would be "good fun" to tour with Atomic Kitten later this year.
Watch the Atomic Kitten 'Whole Again' music video below: